SAN  FRANCISCO 
TRADE  BINDERY 

'  oaplete  Bindery  Strvite 

Phone  YUkon  6-2887 


LIBKAKY    I  ~ 


UNITED   STATES   GEOLOGICAL   SURVEY 

CHARLES  I).  WALCOTT,  DIRECTOR 


A 


TREATISE  ON  METAMORPHISM 


BY 


CHARLES   RICHARD    VAN   HISE 


WASHINGTON 

GOVERNMENT     PRINTING     OFFICE 

1904 


CONTENTS. 


Page. 

I  .KTTKU  OF  TRAXSMITTAL - 

CHAPTER  I. — INTRODUCTION 31 

General  nature  of  alterations 31 

Classification  of  metamorphism 39 

Geological  factors  affecting  the  alterations  of  rocks 40 

Composition 40 

Structures  and  textures 40 

Porosity 40 

Water  and  gaseous  content 41 

Climatic  and  geographic  conditions 41 

Time 41 

Environment 42 

Degree  of  movement 42 

Depth.... 43 

CHAITER  II. — THE  FORCES  OF  METAMORPHISM 45 

Chemical  energy 45 

Gravity 46 

Mechanical  action * 46 

Molecular  mechanical  action 47 

Mass  mechanical  action 49 

Permanent  strain  without  openings 49 

Permanent  strain  with  openings 49 

Permanent  strain  with  closing  of  openings  and  welding 50 

Water  action 50 

Heat  and  light 51 

Sources  of  heat  and  light 51 

The  sun  as  a  source  of  heat  and  light :'. 51 

Heat  derived  from  within  the  earth  by  conduction  or  convection  through  water 

or  magma 53 

Mechanical  action  as  a  source  of  heat 54 

Chemical  action  as  a  source  of  heat 54 

Effects  of  heat  and  light  on  alterations  of  rocks 54 

Direct  effects  of  heat  and  light 55 

Mechanical  effects 55 

Chemical  effects 56 

indirect  effects  of  heat  and  light 56 

General  statements 57 

5 


6  CONTENTS. 

/ 

Page. 

CII.MTER  III. — THE  AGENTS  OK  METAMORPHISM ">S 

General  statement 5s 

Part  I.  Gaseous  solutions 59 

Section  1 .  Chemical  ami  physical  principles  controlling  the  action  of  gases 60 

Gases  |  iresent CO 

The  pressure 61 

The  temperature 61 

Section  2.  Geological  work  c  >f  gases 6L' 

Part  II.  Aqueous  solutions  and  solids 63 

General  considerations 63 

Section  1.  Chemical  and  physical  principles  controlling  the  action  of  ground  water..  65 

Principles  of  solutions  applicable  to  ground  water 65 

Solution  of  gases  in  ground  waters 68 

Gases  present 68 

The  pressure 70 

The  temperature 72 

Solids  in  solution 72 

Solution  of  solids  in  ground  water 72 

Compounds  present 76 

Relations  of  solution  and  pn  ssure 77 

Relations  of  solution  and  temperature 79 

Speed  of  solution 79 

Quantity  of  material  which  may  be  held  in  solution 79 

Relations  of  solution  to  absorption  and  liberation  of  heat 81 

Diffusion 82 

Principles  of  chemical  reactions  applicable  to  ground  waters 84 

General  statement , 84 

Definitions 84 

Dissociation 84 

Hydrolysis 86 

Reactions 87 

Equilibrium 90 

Homogeneous  and  heterogeneous  systems 90 

Nature  and  speed  of  reactions 91 

The  compounds 91 

Strength  of  the  solutions 94 

Mechanical  action 95 

Speed  of  chemical  action 95 

Direct  deformation  effect 95 

SI  rain  without  rupture '. 95 

Strain  with  rupture 98 

Readjustment  of  particles 98 

Indirect  heat  effect 99 

Nature  of  the  chemical  reactions 100 


CONTENTS.  7 

CII.UTKR  III. — THE  AGENTS  OF  METAMOUI-IIISM — Continued.  page. 
Part  II.  Aqueous  solutions  ami  solids — Continued. 

Sirtion  1.  Chemical  and  physical  principles,  etc. — Continue*, 

Principles  ill  chemical  reactions  applicable  to  ground  waters — Continued. 
Nature  and  speed  of  reactions — Continued. 
Mechanical  action — Continued. 

Nature  of  the  chemical  reactions — Continued. 

Smaller  rock  volume  as  the  result  of  solution  and  deposition 

without  change  in  chemical  composition 101 

Recrystallization  and  condensation  without  change  of  min- 
erals    101 

Recrystallization  and  condensation  with  change  of  minerals.  10:2 
Smaller  volume  as  the  result  of  solution  and  redeposition  with 

change  in  chemical  composition 103 

Crystallization  and  condensation  of  amorphous  compounds.  103 

Recrystallization   and   condensation   of    crystallized    com-  J 

pounds 103 

General  statements 104 

Heat 105 

Relations  of  chemical  action,  mechanical  action,  and  heat 110 

Precipitation  113 

I 'nvipitation  by  change  of  pressure 1 14 

Precipitation  by  change  of  temperature 1 15 

1'recipitation  by  reactions  between  aqueous  solutions 116 

Precipitation  by  reactions  between  aqueous  solutions  and  gases 1  Hi 

I  recipitation  by  reactions  between  solutions  and  solids 120 

Section  2.  Circulation  and  work  of  ground  water 123 

I'niversal  presence  of  water  in  rocks 123 

Pore  space  of  rocks 125 

Circulation  of  ground  water 129 

Openings  in  rocks 129 

Form  and  continuity  of  openings 129 

Sixc  of  openings 134 

Supereapillary  openings .__: 137 

Capillary  openings 138 

Rubcapillary  openings 143 

Percentage  of  openings,  or  pore  space 146 

Forces  producing  water  circulation 1 46 

( !  ravit  y 146 

Heat 147 

Gravity  and  heat 148 

Mechanical  action 149 

Molecular  attraction 150 

Vegetation 152 

General  statements..  152 


8  CONTENTS. 

CHAPTER  III. — THE  AGENTS  OK  METAMORI-HISM — Continued.  Page. 
Part  II.  Aqueous  solution."  ami  solids — Continued. 

Section  2.  Circulation  and  work  of  ground  water — Continued. 
Circulation  of  ground  water — Continued. 

The  factor  opposing  water  circulation 153 

General  statements 154 

Geological  work  of  ground  water '  156 

CHAPTER  IV. — THE  ZONES  AMI  BELTS  OF  METAMOKIMIISM 151) 

<  M-neral  considerations 159 

Zone  of  katamorphism 160 

Belt  of  weathering 163 

Belt  of  cementation 164 

Belts  of  weathering  and  cementation  contrasted : 166 

Zone  of  anamorphism .' 167 

Relations  of  zones  of  katamorphism  and  anamorphism 170 

General  considerations 186 

Relations  of  zones  of  katamorphism  and  anamorphism  to  zones  of  fracture  and  flowage  . .  187 

Upper  limit  of  zone  of  flowage 187 

CHAPTER  V. — MINERALS 192 

Section  1.  Chemical  and  mineral  composition  of  the  known  crust  of  the  earth 192 

Section  2.  General  nature  of  alterations 202 

Alteration  without  change  in  chemical  composition _ . 202 

Molecular  rearrangement 202 

Simple  recrystallization 202 

Alteration  with  change  in  chemical  composition 202 

Alteration  without  addition  or  subtraction  of  material 203 

Alteratipn  with  addition  or  subtraction  of  material 2u:> 

General  statements 206 

Section  3.  Rock-making  minerals 207 

Manner  of  treatment 207 

General  statements 207 

Native  elements 212 

Graphite 212 

Occurrence 212 

Alterations 21 2 

The  sulphides 212 

Pyrrhotite,  pyrite,  and  marcasite 213 

Occurrence 213 

Alterations 214 

The  fluorides 216 

Fluorite _ 216 

Occurrence 216 

Alterations 216 

The  oxides 217 

Quartz 21 7 

Occurrence 21 7 


CONTENTS.  9 

CHAPTER  V. — MINERALS — Continued.                                                  y  Page. 
Section  3.  Rock-making  minerals — Continued. 
The  oxides — Continued. 
Quartz — Continued. 

Modifications 218 

Tridymite 220 

Occurrence 220 

Modifications 220 

Opal 221 

Occurrence 221 

Modifications 221 

Chert,  chalcedony,  etc 222 

Hematite  group 223 

Corundum,  hematite,  and  ilmenite 223 

Corundum 223 

Occurrence 223 

Alterations 223 

Hematite 225 

Occurrence 225 

Alterations 226 

Ilmenite 227 

Occurrence 227 

Alterations 227 

Spinel  group 228 

Spinel,  magnetite,  and  chromite 228 

Spinel 228 

Occurrence 228 

Alterations 228 

Magnetite 229  ' 

Occurrence 229 

Alterations 229 

Chromite 229 

Occurrence 229 

Alterations 230 

Entile  group 230 

Rutile,  octahedrite,  and  brookite 230 

Occurrence 230 

Alterations 230 

Diaspore  group 231 

Diaspore  and  limonite 231 

Diaspore 232 

Occurrence 232 

Alterations 232 

Limonite '. 232 

Occurrence 232 

Alterations 233 

i 


10  CONTENTS. 

CII.UTEK  V. — MINERALS — Continued.  Page 
Section  3.   Rock-making  mineral.-' — Continued. 
The  oxides — Continued. 

Ilrucite  group 

Brucite  and  gibbsite 

Brucite '-•* 

Occurrence 235 

Alterations -'•'•' 

Gibbsite 235 

Occurrence 235 

Alterations 2:!5 

The  carbonates 236 

Calcite  group 237 

Calcite,  dolomite,  ankerite,  parankerite,  magnesite,  and  siderite 237 

Calcite 237 

Occurrence 237 

Alterations 

Dolomite 240  . 

Occurrence '-- 240 

Alterations -41 

Ankerite  and  parankerite 242 

Occurrence 242 

Alterations 242 

Magnesite -  -  -  243 

Occurrence 243 

Alterations 243 

Siderite 244 

Occurrence 244 

Alterations 244 

Aragonite  group 245 

Aragonite 245 

Occurrence 245 

A  Iterations 245 

The  silicates 24(> 

Glass 246 

Occurrence 247 

Evidence  that  devitrification  takes  place 247 

Scale  of  devitrification 247 

Rate  of  devitrification 248 

Devitrification  in  the  two  zones 24i> 

Minerals  produced <- 251 

Heat  and  volume  relations 251 

Feldspar  group 

Munoclinic  or  peeudtimonoclinic 253 

Orthoclase,  microcline,  and  anorthoclase 253 

Orthoclase  and  microcline 253 


CONTENTS.  1 1 

CHAPTER  V. — MINERALS — Continued.  Page. 
Sections.   Rock-making  minerals — Continued. 
The  silicates — Continued. 

Feldspar  group — Continued. 
Monoclinic  or  pseudonoonoclinic — Continued. 
Oltboclase,  microcline,  and  anorthoclase — C'oiitinued. 
Orthoclase  and  microcline — Continued. 

Occurrence 253 

A  Iteral  ions'. 253 

Anorthoclase 257 

Occurrence L'.">7 

Alterations 258 

Triclinic LTi!) 

Albite,  oligoclase,  andesine,  labradorite,  bytownite,  and  anorthite 259 

Occurrence 259 

A  Iterations 260 

Leucite  group 266 

Leucite 26(3 

Occurrence 266 

Alterations 266 

Pyroxene  group 267 

Orthorhombic  pyroxenes 267 

Enstatite,  bronzite,  and  hypersthene 267 

Occurrence 268 

Alterations 268 

Monoclinic  pyroxenes 271 

IMopside,  sahlite,  hedenbergite,  augite,  acmite,  spodumene,  wollastonite, 

and  pectolite 27 i 

Occurrence 272 

Alterations  of  the  diopside-augite  series 273 

Alterations  of  pyroxenes  other  than  the  diopside-augite  series 280 

Amphibole  group 281 

Orthorhombic  amphiboles. 281 

Anthophyllite  and  gedrite -. 281 

Occurrence 281 

Alterations ; 282 

Monoclinic  amphiboles 283 

Tremolite,   actinolite,    cummingtonite,    griinerite,    hornblende,   glauco- 

phane,  riebeckite,  and  arf vedsonite 283 

Occurrence 28'5 

Alterations ^s;, 

lolite  (cordierite) 291 

Occurrence 291 

Alterations 291 

Nephelite  group 2112 

Nephelite  and  cancrinite 2UL> 


12  CONTENTS. 

CHAPTER  V. — MINERALS — Continued.  i^,,. 
Section  3.  Rock-making  minerals — Continued. 
The  silicates — Continued. 

.Jsephelite  group — Continued. 

Nephelite  and  cancriniti — Continued. 

Nephelite 292 

Occurrence 292 

Alterations 292 

Cancrinite 2ii4 

Occurrence 294 

Alterations 294 

Sodalite  group '. 295 

S<  idalite,  haiiynite,  and  noselite 295 

Sodalite . 295 

Occurrence 295 

Alterations 2!)5 

Haiiynite  and  noselite 297 

Occurrence 2H7 

Alterations 298 

Garnet  group 299 

Grossularite,  pyrope,  almandite,  spessartite,  melanite,  and  uvarovite 299 

Occurrence 300 

Alterations 302 

Chrysolite  group 308 

Forsterite,  olivine,  and  fayalite 308 

Occurrence 308 

Alterations 308 

Srapolite  group 311 

Mcionite,  wernerite,  and  inarialite 311 

Occurrence 312 

Alterations 312 

Melilite 314 

Occurrence 314 

Alterations 314 

( iehlenite 3]  4 

Occurrence 314 

Alterations 314 

Vesuvianite ;;15 

Occurrence 315 

Alterations 315 

Zircon  gn nip 315 

Occurrence 315 

Alterations 315 

Aluminum-silicate  group 316 

Topaz,  andalusite,  sillimanite,  and  oyanite :;i ii 

Occurrence 316 

Alterations 31g 


CONTENTS.  13 

ClI.UTER    V. — MlXKK.U.S — Continued.  Page. 

Section  3.   Rock-making  minerals — Continued. 
The  silicates— ( 'ontinued. 

Epidiite  group 320 

Zoisite,  epidote,  piedmontite,  and  allanite 320 

Occurrence 320 

Alterations 322 

Axinite 323 

Occurrence 323 

Alterations 324 

Prehiiite 324 

Occurrence 324 

Alterations 324 

Humite  group 325 

Chondrodite,  humite,  and  clinohumite 325 

Occurrence 325 

A  Iterations 325 

Tourmaline 326 

Occurrence 326 

A  Iterations 326 

Staurolite 327 

Occurrence 327 

Alterations 327 

Zeolite  group 329 

Thomsonite,  hydronephelite,  natrolite,  mesolite,  scolecite,  analcite,  apophyl- 
lite,  epistilbite,  heulandite,  stilbite,  phillipsite,  harmotome,  gismondite, 

chabazite,  gmelinite,  and  laumontite 329 

Occurrence 331 

Alterations 333 

Mica  group 336 

Muscovite,  paragonite,  biotite,  and  phlogopite 336 

Muscovite 336 

Occurrence 336 

Alterations -.' 337 

I  'arugonite 338 

Occurrence 338 

Alterations 338 

Biotite - 339 

( >ccurrence 339 

Alterations 339 

Phlogopite '. 343 

Occurrence 343 

A  Iterations 343 

Clintonite  group 344 

Margarite,  chloritoid,  and  ottrelite 344 

Occurrence :'.44 

Alteration-  . .  345 


14  CONTENTS. 

CHAPTER  V. — MINERALS — Continued.  Page. 
Section  3.  Rock-making  minerals — Continued. 
The  silicates — Continued. 

Chlorite  group 345 

Ameaite,  corundophilite,  prochlorite,  clinochlore,  and  penninite 345 

Occurrence 346 

Alterations 347 

Serpentine-talc  group 

Serpentine  and  talc 348 

Serpentine 

Occurrence 349 

Alterations 349 

Talc 350 

Occurrence 350 

Alterations 351 

Glauconite 351 

^                   Occurrence 351 

Alterations 351 

Kaolin  group 352 

Occurrence 352 

Alterations :.  352 

Summary  of  alteration  of  silicates 352 

The  titanates 354 

Titanite  and  perovskite 354 

Titanite 354 

Occurrence 354 

Alterations 355 

Perovskite 355 

Occurrence 355 

Alterations 355 

The  phosphates 356 

Apatite 356 

Occurrence 356 

Alterations 356 

The  sulphates 357 

Anhydrite  and  gypsum 357 

Anhydrite 357 

Occurrence 357 

Alterations 357 

Gypsum 357 

Occurrence 357 

Alterations 358 

Section  4.  General  statements 359 

Physical-chemical  factors  upon  which  nature  of  alterations  depends 359 

Chemical  composition 359 


CONTENTS.  15 

CHAPTER  V. — MINERALS — Continued. 

Section  4.  <  ieneral  statements — Continued. 

Physical-chemical  factors  upon  which  nature  of  alterations  depends — Continued. 

Chemical  composition 359 

Chemical  composition  of  adjacent  minerals 359 

Chemical  composition  of  circulating  solutions 359 

Specific  gravity 360 

Symmetry 360 

Specific  gravity  and  symmetry 361 

Heat  reactions 362 

Pressure  and  volume 363 

Reversible  reactions 366 

Sections.  Tables 369 

Table  A.  Sources  of  minerals 369 

Table  B.  Alteration  products  of  minerals 372 

Table  C.  Chemical  reactions  and  volume  changes ._. 375 

Table  D.  Classification  of  alterations,  with  volume  changes 395 

CHAPTER  VI. — THE  BELT  OF  WEATHERING 409 

Belt  of  weathering  denned -, 409 

Form  of  level  of  ground  water 411 

Amount  and  source  of  water  in  belt  of  weathering 413 

The  circulation 416 

Downward  movements  of  water 417 

Upward  movements  of  water 419 

Molecular  attraction 419 

Vegetation  422 

Variation  in  level  of  ground  water 428 

Precipitation,  seepage,  and  evaporation 423 

Uplift  and  subsidence 426 

Denudation  and  valley  tilling 426 

Influence  of  man 427 

Barometric  pressure 428 

Temperature 

General  statements 429 

Metamorphism  in  the  belt  of  weathering 429 

Variable  materials  and  conditions  of  belt  of  weathering 429 

Mechanical  work 431 

Water,  ice,  and  wind 432 

Change  in  temperature ; 

Change  from  water  to  ice 440 

Plants 444 

Lichens,  mosses,  etc 

Cacti 445 

Grasses,  grains,  and  vegetables 445 

Shrubs  and  trees 445 


16  CONTENTS. 

C'HAJTER  VI.- — THE  BELT  OF  WEATHERING — Continued.  Page. 
Metamorphism  in  the  belt  of  weathering — Continued. 
Mechanical  work — Continued. 

Animals 447 

Earthworms 44S 

Aiits,  termites,  and  other  insects 448 

The  larger  burrowing  animals 449 

Man : 450 

General  statements 451 

Chemical  work 4.51 

The  agents 45:.' 

Plants. . 451' 

Plants,  alive 452 

Plants,  dead,  and  bacteria 455 

Animals 456 

Animals,  alive 456 

Animals,  dead,  and  bacteria 457 

Work  of  solutions 457 

Joint  work  of  agents  of  weathering 461 

Oxidation 461 

Oxidation  of  organic  compounds i 461 

( )xidation  of  carbon  and  hydrogen 461 

Oxidation  of  nitrogen 465 

Oxidation  of  inorganic  compounds 466 

Iron 467 

Sulphur 468 

General  statements 469 

Carbonation 473 

Hydration  and  dehydration 481 

Oxidation,  carbonation,  and  hydration 483 

Solution 484 

Deposition . 487 

General  statements 487 

Contact  metamorphism 488 

Direct  contact  effect 489 

Indirect  contact  effect,  or  work  of  fumaroles  and  solfataras 490 

Relations  of  disintegration  to  decomposition  and  solution 494 

Regions  favorable  to  prominence  of  disintegration '  496 

Arid  regions 496 

Regions  of  high  latitude 498 

Regions  of  marked  topographic  relief 499 

Regions  of  sparse  plants  and  animals 500 

Regions  near  the  sea 500 

Regions  favorable  to  prominence  o  f  decomposition 501 

Humid  regions 501 

Regions  of  low  latitude 502 


CONTENTS.  17 

CHAPTER  VI. — THE  BELT  OF  WEATHERING — Continued.  Page. 
Metamorphism  in  the  belt  of  weathering — Continued. 

Relations  of  disintegration  to  decomposition  and  solution — Continued. 
Regions  favorable  to  prominence  of  decomposition — Continued. 

Regions  of  moderate  topographic  relief 502 

Regions  of  abundant  plants  and  animals 503 

Regions  remote  from  the  sea 504 

General  statements 504 

Change  in  chemical  composition  of  the  rocks 507 

Order  of  decomposition  of  the  minerals,  and  the  end  products 518 

Total  gains  and  losses  in  weathering,  and  changes  in  volume 522 

Emphasis  and  retention  of  structures  and  textures 524 

Obliteration  of  structures  and  textures 526 

Surfaces  of  weathering 527 

Depth  and  degree  of  weathering 529 

Rate  of  weathering 532 

Forces  and  agents  at  work  in  weathering 532 

Materials  weathered 532 

Chemical  composition 532 

Mineral  composition 533 

State  of  aggregation 533 

Thickness  of  the  belt  of  weathering 534 

Stage  of  weathering 535 

Distribution  of  dissolved  materials ' 536 

Material  abstracted  by  plants 537 

Material  transferred  to  belt  of  cementation 538 

Material  permanently  abstracted  by  run-off 538 

Material  dissolved,  transported,  and  reprecipitated  in  belt  of  weathering 539 

Concentration  at  and  near  the  surface 543 

Concentration  by  underground  circulation 543 

Concentration  by  circulation  mainly  confined  to  belt  of  weathering 544 

Concentration  by  circulation  extending  into  belt  of  cementation 550 

Concentration  by  overground  circulation 551 

Distribution  of  residual  materials 554 

Relations  of  belt  of  weathering  to  sedimentary  rocks 555 

Belt  of  weathering  the  source  of  sedimentary  rocks 555 

Material  transported  in  suspension 556 

Material  transported  in  solution 557 

Material  transported  in  suspension  and  solution 558 

Rocks  produced  from  material  of  belt  of  weathering  without  transportation  to  the   . 

sea 559 

Transition  between  belt  of  weathering  and  belt  of  cementation 560 

CHAPTER  VII. — THE  BELT  OP  CEMENTATION 562 

Belt  of  cementation  denned 562 

Boundaries  of  belt  of  cementation 565 

MON   XLVII — 03 2 


18  CONTENTS. 

CHAPTER  VII. — THE  BELT  OP  CEMENTATION— Continued.  Pace 

Condition  of  water  in  belt  of  cementation iiiid 

Amount  of  water  in  belt  of  cementation "xi'.i 

Circulation  of  water  in  belt  of  cementation 571 

Vigor  of  circulation 57! 

Character  of  circulation ">72 

Limiting  formations 57t> 

Gravity 578 

Increase  of  temperature  with  depth 57s 

Relative  lengths  of  vertical  and  horizontal  components 57!i 

Preferential  use  of  large  channels 580 

Resultant  circulation 582 

General  statement 589 

Temperature  of  entering  and  issuing  water,  and  transfer  of  heat 5s;i 

Variable  materials  and  conditions  of  belt  of  cementation 594 

Work  in  belt  of  cementation 59 1 

Mechanical  work 594 

Consolidation 5ii5 

Strain  within  elastic  limit 597 

Strain  beyond  elastic  limit 5!i'.i 

Chemical  work (id- 
Chemical  changes (602 

Oxidation 604 

Carbonation 608 

Hydration 612 

Solution  and  deposition 612 

Quantitative  relations  between  solution  and  deposition 613 

Resultant  processes (>l  7 

Cementation 617 

Cementing  substances 621 

Oxides 622 

Silica 622 

Iron  oxides 623 

Aluminum  oxides 624 

Carbonates 624 

Calcite  and  dolomite 624 

Siderite 625 

Silicates 625 

Sulphides 627 

Distribution  of  elements  in  cementing  minerals 627 

Distribution  of  cementing  minerials 628 

Causes  of  cementation <;L".< 

Expansion  reactions 631 

Contributions  from  igneous  rocks  ...'. 634 

Selective  precipitation 634 

Diffusion 636 

Conclusion  ..  639 


CONTENTS.  19 

• 

VII.—  THE  BELT  OF  CEMENTATION— Continued.  Page. 

Work  in  licit  of  cementation — Continued. 
Chemical  work — Continued. 

Resultant  processes — Continued. 

Metasomatism   640 

Definition 640 

Extent  of  process 640 

Conditions  favorable  to  metasomatism 641 

Minerals  produced 642 

(irowth  of  large  individuals  and  preservation  of  textures 643 

Segregation  of  individual  minerals 645 

Igneous  work 646 

Injection 646 

Combinations  and  relations  of  mechanical  work,  chemical  work,  and  igneous  work..  653 

Changes  of  chemical  composition 655 

CHAPTER  VIII. — THE  /O.N-E  OF  ANAMORPHISM 657 

Definition  of  z<  me 657 

Condition  of  water 65S) 

i  Quantity  of  water 661 

Circulation  of  water 661 

Variable  materials  and  conditions 6(58 

Work  in  zone  of  anamorphism 670 

Mechanical  work 670 

Welding 670 

Strain  within  elastic  limit 671 

Strain  beyond  elastic  limit 673 

Chemical  work 675 

Chemical  changes 676 

Deoxidation 676 

S  i  1  i  cation 677 

Dehydration i;7!> 

Solution  and  deposition  / 680 

Resultant  processes 681 

Cementation _• 681 

Metasomatism 682 

Minerals  formed 683 

Alterations  in  connection  with  mass-mechanical  action 685 

Recrystallization 686 

Facts  of  recrystallization  686 

Theory  of  reerystallization '       690 

Recrystallization  lags  behind  deformation 696 

Conclusion 698 

Alterations  under  mass-static  conditions 698 

Development  of  porphyritic  textures 699 

Regeneration  of  mineral  particles 705 


20  CONTENTS. 

t 

(  HAITEH  VIII. — TitK  /ONE  OF  ANAMORPHISM — Continued.  Page. 
\\"<>rk  in  /om-  of  anamorphism — Continued. 

Igneous  work 707 

Injection 707 

Manner  of  intrusion 708 

Eesultant  metamorpbiam 7]] 

Factors  controlling  metamorphism 711 

Size  of  intrusive  masses 711 

The  temperature 712 

Amount  of  water  present 712 

Composition  of  intrusive  and  intruded  rocks 713 

Metamorphic  effects 716 

Structures 716 

The  minerals 717 

Pegmatites _ 720 

Fusion  and  absorption 728 

Combinations  and  relations  of  the  various  processes 736 

Relations  of  granulation  and  recrystallization 737 

Character  of  material 738 

Temperature 740 

Pressure  and  rapidity  of  deformation 741 

Water  content 741 

Rock  flowage 748 

Meaning  of  rock  flowage 74S 

Conclusion 759 

Meaning  of  rock  cleavage 760 

Effect  of  rock  flow  on  textures  and  structures •    760 

Rock  flowage  and  mashing 762 

Changes  in  chemical  composition 764 

Relations  of  zone  of  anamorphism  to  zone  of  katamorphism 766 

Comparative  energy  required   for    deformation   in    zones  of   katamorphigm   and   ana- 
morphism   - 769 

Conclusion 774 

CHAPTER  IX. — ROCKS 77") 

Use  of  some  general  terms  applied  to  metamorphic  rocks 77ti 

Meta 776 

Apo  776 

Slate  and  schist 778 

Slate 778 

Schist .' 779 

Gneiss 782 

General  statements 783 

Sedimentary  rocks 784 

Nonfragmental  class ' 787 

Nitrate  order 787 

Niter  familv 7S7 


CONTENTS.  21 

CHAPTER  IX. — ROCKS— Continued.  Page. 
Sedimentary  rocks — Continued. 

Nonf ragmen tal  class — Continued. 

Sulphate  order 788 

(iypsnin  and  anhydrite  family 788 

Chloride  order 789 

Uork-salt  family 789 

Carbonate  order 791 

Calcium-magnesium  carbonate  family  and  metamorphosed  equivalents 791 

Limestones 791 

Source  of  material  of  limestones 791 

Organic  precipitates 792 

Chemical  precipitates 793 

Springs  and  streams 793 

Inland  seas  with  no  outlets 793 

Possible  chemical  precipitates  in  the  ocean  or  in  seas  con- 
nected with  the  ocean 793 

Metamorphigm  of  organic  and  chemical  calcium  carbonate  deposits. .  795 

Dolomite 798 

Origin  of  dolomite 798 

Dolomite  due  to  replacement  of  calcium  by  magnesium 798 

Conclusion 802 

How  and  why  dolomitizatioii  occurs 802 

Dolomitization  before  limestone  emerges  from  the  sea 802 

Dolomitization  after  limestone  emerges  from  the  sea 804 

Marble 808 

Cherty  limestones,  cherty  dolomites,  and  eherty  marbles 816 

Silicated  marbles 820 

Silicate  rocks 822 

General  statements 823 

Iron-bearing  carbonate  family  and  metamorphosed  equivalents 823 

Siderite,  ankerite,  and  parankerite 823 

Origin  of  siderite,  ankerite,  and  parankerite 824 

Ferruginous  shales,  ferruginous  cherts,  and  jaspilites 829 

Ferruginous  shales 830 

Ferruginous  cherts 830 

Jaspilites 831 

Actinolitic  and  griineritic  marbles 833 

Actinolite-magnetite-quartz  rocks  and  grunerite-magnetite-quartz  rocks.  834 

General  statements 841 

Oxide  order 842 

Iron-oxide  family 342 

Limonite 842 

Hematite 843 

Magnetite 845 


22  CONTENTS. 

CHAPTER  IX. — ROCKS — Continm-cl.  !•;,,.,. 
Sedimentary  rocks — Continued. 

Nonfragmental  class — Continued. 
Oxide  order — Continued. 

Silica  family 847 

Chert 847 

Rearrangement  of  chert sin 

Fragmental  class 85:! 

Psephite  order s:>.; 

Pebble,  gravel,  and  bowlder  deposita sr>:; 

Conglomerates S.">5 

Schist-conglomerate  and   gneiss-pgephite,    or   conglomerate-schist    and 

]isc|  ihitc-jruciss s:>7 

Psammite  order x  ii  i 

Quartz-sand  family M;O 

Quartz-sand  mck 860 

Sandstone sii4 

Quart/.ite sii5 

Schist-quartzite  or  quartzite-schist SliS 

Quartz-feldspar-sand  family 870 

Quartz-feldspar  sand S70 

Arki  ise 874 

Schist-arkose  and  gneiss-arkose,  or  arkose-schist  and  arkose-gneiss 875 

Ferromagnesian-sand  family S77 

Ferromagnesiao  sands 877 

Grits , 879 

Gray  wacke 880 

Slate-gray vvacke,  schist-graywacke,  ami  iznoiss-graywacke;  orgraywacke- 

.    slate,  gray wacke-schist,  and  graywacke-gneiss 883 

Pelite  order 886 

Mud  family 886 

Shale  family 8WL' 

Slate-pelite,  schist-pelite,  and  gneiss-petite;  or  pelite-slate,  pelite-schist, 

and  pelite-gneiss s'.i  1 

^Development  of  minerals  of  slates 898 

Development  of  minerals  of  schists  and  gneisses SIM 

Igneous  rocks 1104 

CHAPTER  X. — THE  RELATIONS  OF  METAMOHPHISM  TO  STRATIOKAPHV !)07 

Introductory !)()7 

Discrimination  between  metamorphosed  sedimentary  and  metamorphosed  igneous  rocks.  908 

Cases  of  confusion 909 

Criteria  for  discrimination '.Hi' 

Relations  of  metamorphic  sedimentary  rocks  to  stratigraphy !U  7 

Variation  in  metamorphism : 917 

Upon  what  variations  are  dependent 917 

Resulting  variations 918 


CONTENTS.  23 

CHAPTER  X. — THK  RELATIONS  OK  MBTAMORPHISM  TO  STRATIGRAPHY — Continued.  p«se 

Ui 'hit inns  of  igneous  roeks  to  stratigraphy 922 

Relations  of  rock  flowage  to  mountain  making 924 

<  'IIAITER  XI. — RELATIONS  OF  METAMOKPHISM  TO  THK  DISTRIBUTION  OF  THE  CHEMICAL  ELEMENTS.  932 

Composition  of  the  iithoephere 932 

Table  of  analyses  of  igneous  and  crystalline  rocks 934 

Table  giving  symbols,  atomic  weights,  and  proportions  of  the  twenty-three  most 

abundant  elements  in  the  outer  10  miles  of  the  earth 936 

Table  showing  the  amounts  of  the  eleven  most  common  oxides  of  the  lithosphere,  ;is 

estimated  in  1891  and  1900 937 

Composite  analyses  of  sedimentary  rocks 938 

Constituents  of  meteorites 94."i 

Redistribution  of  the  chemical  elements 947 

Oxygen 948 

Sulphur 957 

Silicon !i.V.i 

Carbon s      962 

Amount  of  carbon 962 

Segregation  of  carbon 964 

Segregation  by  carbonation 964 

Segregation  in  carbonaceous  deposits 966 

Sources  of  segregated  carbon 967 

Titanium 974 

.  Phosphorus H~r> 

Chlorine : 978 

Nitrogen 980 

Hydrogen 981 

Aluminum  (is:; 

I  ron 986 

Manganese 989 

Calcium 990 

Magnesium 992 

Sodium  996 

Potassium 999 

Barium,  strontium,  chromium,  nickel,  lithium,  fluorine,  bromine 1002 

General  statements 1 1002 

CHAPTER  XII. — THE  RELATIONS  OK  METAMORPHISM  TO  ORE  DEPOSITS 1004 

Part  I.  General  principles 1004 

Introductory 1004 

Classification  of  ore  deposits 1005 

Deformation  of  the  lithosphere 1005 

Zone  of  fracture,  or  zone  of  katamorphism 1005 

Openings  of  zone  of  fracture 1006 

Form  and  continuity  of  openings 1007 

Size  of  openings 1008 

Volume  of  openings 1008 


24  CONTENTS. 

CHAPTER  XII. — THE  RELATIONS  OF  METAMORPHISM  TO  ORE  DEPOSITS — Continued.  Page. 
Part  I.  General  principles — Continued. 

Deformation  of  the  lithosphere — Continued. 

Zone  of  fracture,  or  zone  of  katamorphisin — Continued. 

Chemical  reactions 1008 

Zone  of  combined  fracture  and  flowage 1009 

Zone  of  flowage,  or  zone  of  anamorphism 1011 

Openings  of  zone  of  flowage 1012 

Reactions  of  zone  of  flowage 1012 

Relations  between  zones  of  deformation 1012 

Effects  of  deformation  and  chemical  changes  upon  temperature 1013 

Volcanism 1014 

Circulation  and  work  of  solutions 1017 

Circulation  of  gaseous  solutions 1018 

Circulation  in  belt  of  weathering 1018 

Circulation  in  belt  of  cementation 1019 

Circulation  in  zone  of  anamorphism 1020 

Circulation  of  aqueous  solutions 1021 

Circulation  in  zone  of  fracture,  or  zone  of  katamorphism 1022 

Belt  of  weathering 1023 

Belt  of  cementation 1024 

Circulation  in  zone  of  combined  fracture  and  flowage 1 028 

Circulation  in  zone  of  flowage,  or  zone  of  anamorphism 1029 

Source  of  the  metals..                                           1030 

«      i 

Part  II.  Segregation  of  ores 1036 

General  statements 1036 

Division  A.  Ores  produced  by  processes  of  sedimentation 1037 

Ores  formed  by  chemical  precipitation 1037 

Ores  formed  by  mechanical  concentration 1038 

Metamorphic  alterations  of  sedimentary  ores 1039 

Division  B.  Ores  produced  by  igneous  processes 1043 

Division  C.  Ores  produced  by  processes  of  metamorphism 1052 

Group  A.  Ores  deposited  by  gaseous  solutions 1052 

Group  B.  Ores  deposited  by  aqueous  solutions  . .  .• 1058 

I.  Source  of  aqueous  solution* 1065 

II.  Source  of  metals  for  ores  deposited  from  aqueous  solutions 1069 

III.  Work  of  aqueous  solutions  in  segregating  ores 1072 

Subclass  1.  Ores  precipitated  from  ascending  aqueous  solutions 1072 

Solution  of  the  metals 1073 

Transportation  of  the  metals 1075 

Precipitation  of  the  metals 1081 

Precipitation  by  decrease  of  temperature  and  pressure 1081 

Precipitation  by  mingling  of  solutions 1082 

Precipitation  by  reactions  between  solutions  and  solids 1086 

General  statements . .  1088 


CONTENTS.  25 

\ 

CHA"PTER  XII. — THE  RELATIONS  OK  METAMORPHISM  TO  ORE  DEPOSITS — Continued.  Page. 
Part  II.  Segregation  of  ores — Continued. 

Division  C.  Ores  produced  by  processes  of  metainorphism — Continued. 
< ;  n  >up  B.  Ores  deposited  by  aqueous  solutions — Continued. 

III.  Work  of  aqueous  solutions  in  segregating  ores— Continued. 

Sulirlass  1.  Ores  precipitated  from  ascending  aqueous  solutions — Cont'd. 

Compounds  deposited  by  ascending  solutions 1088 

Metals 1089 

Gold 1089 

Solution 1089 

Precipitation  .' 1091 

Silver 1099 

Solution 1099 

Precipitation 1100 

Copper 1101 

Solution 1101 

Precipitation 1101 

Sulphides 1104 

Solution  of  sulphides 1106 

Precipitation  of  sulphides 1108 

Precipitation  of  sulphides  transported  as  such 1109 

Precipitation  of  sulphides  transported  as  oxidized  salts.  1110 

General  statements 1117 

Tellurides 1119 

Oxides 1125 

Magnetite 1126 

Zincite 1126 

Franklinite .• 1126 

Hematite 1126 

Cassiterite 1127 

Carbonates 1128 

Silicates • 1129 

Criteria  for  discriminating  deposits  of  the  deep  circulation 1 132 

General  statements _.- 1138 

Subclass  2.  Ores  precipitated  from  ascending  and  descending  aqueous 

solutions 1139 

Association  of  lead,  zinc,  and  iron  compounds , 1144 

Facts  of  occurrence 1144 

Second  concentration 1147 

Oxidized  ores 1147 

Sulphide  ores 1 148 

Galena 1148 

Sphalerite 1151 

Mareasite  and  py rite 1152 

General  statements . .  1153 


26  CONTENTS. 

CHAJTKH  XII. — THE  RELATIONS  OF  MKT  vMoi:i'iiis\i   TO  OKI-:  PKPOSITS — Continued,  r'n:. . 
Part  II.  Segregation  of  ore- — ( 'ontinued. 

Division  ('.   Ores  produced  l>y  processes  of  metamorphism — Continued. 
Group  15.   Ores  deposited  l>y  aqueous  solutions — Continued. 

III.  Work  of  aqueous  solutions  in  segregating  ori's — Continued. 

Subclass  2.  Ores  precipitated  from  ascending  and  descending  aqueous 
solutions — Continued. 

Association  of  copper  and  iron  compounds 1 1  ~>S 

Association  of  silver  and  gold  with  base  metals 1  Kit! 

Silver IKili 

(i.)ld 116!) 

Concentration  by  reaction  upon  sulphides  compared  with  metallur- 
gical concentration 11 74 

Other  reactions  of  descending  solutions .' 117") 

Second  concentration  favored  by  large  openings  near  the  surface  ...  1177 

Depth  of  effect  of  descending  waters 117!) 

Illustrations  of  secondary  enrichment  and  diminution  of  richness 

with  depth 1182 

General  statements 1189 

Subclass  3.  Ores  precipitated  from  descending  aqueous  solutions 1193 

Iron  ores 119:; 

Manganese  ores 1 198 

IV.  Special  factors  affecting  the  concentration  of  ores 119!) 

Variations  in  porosity  and  structure 1200 

Distribution  and  size  of  openings 1201 

.  Complexity  of  openings 1202 

Preexisting  channels  and  replacements 1203 

Impervious  strata  at  various  depths 1207 

Pitching  troughs  and  arches 1211 

General  statements 121(5 

Character  of  topography 1217 

Effect  of  vertical  etement 1217 

Effect  of  horizontal  element 1218 

Physical  revolutions 1 221 

V.  General  statements 122:> 

VI.  Ore  shoots 122:; 

General  statements 1230 

Summary  and  conclusion 1232 

INDEX 1 245 


I L  L  US T  R  A  T I  0  N  S . 


Page. 

I'I.ATJO   I.   Fail-view  Dome,  Sierra  Nevada,  from  the  north 43l> 

II.   -1,   Parallel   veins  of    i-alcite,   (ireat   Basin;    B,   Biotitie   granite,   showing  garnet 

surrounded  by  lenticular  areas  deficient  in  in. n-bearing  minerals 700 

III.  Photomicrographs  of  metamorphic  textures , 704 

IV.  Textures  of  limestones  and  marble 796 

V.   Textures  of  metamorphosed  marbles 810 

VI.  Photomicrographs  nf  limestone  and  marbles 814 

Vlf.   Photomicrographs  of  iron-bearing  rocks SMii 

VIII.  A,  Unaltered  Newark  conglomerate  from  Virginia,  after  Keith;  B,  Schist  conglom- 
erate from  Felch  Mountain  district,  Michigan 858 

IX.  Photomicrographs  of  sandstone  and  quartzitoa 872 

X.   Photomicrographs  of  gray \vackes 888 

XI.   Photomicrographs  of  pelites 902 

XII.   .1,  Vein  quart/,  Banner  mine,  California;  B,  Secondary  galena  and  blende  in  ores 

from  Missouri 1156 

XIII.   Iron-ore  deposits  in  pitching  troughs 1196 

FK;.    1.  Change  of  volume  resulting  from  solution,  and  relations  of  solution  and  pressure 78 

•J.   Quantitative  relations  between  solution  and  temperature 80 

:>.  Triangular  en  >s-°  sections  of  pore  space 132 

4.   Spheres  packed  in  the  most  compact  manner  possible 133 

.">.   Relations  of  level  of  ground  water  to  topography  and  to  surface  drainage 410 

6.  Effect  of  unequal  heating  of  the  surface  of  a  rock 434 

7.  Ideal  horizontal  section  of  the  flow  of  ground  water  from  one  well  to  another 570 

8.  Ideal  vertical  section  of  the  llo\v  of  ground  water  from  one  well  to  another 571 

!'.    Ideal  vertical  section  of  the  flow  of  ground  water  entering  at  one  point  on  a  slope  and 

issuing  at  a  lower  point 573 

10.  Ideal  vertical  secii.  m  of  the  flow  of  ground  water  entering  at  three  points  and  issuing 

at  a  single  point •_ 574 

11.  Ideal  vertical  section  of  the  How  of  ground  water  entering  at  many  points  along  a 

slope  and  issuing  at  a  single  point  at  a  lower  elevation ">7.i 

12.  Ideal  section  illustrating  the  chief  requisite  conditions  of  artesian  wells 577 

13.  Part  of  a  thin  section  of  a  quartz-schist  showing  liquid-  and  gas-filled  cavities  of  a 

secondary  nature;  Black  Hills,  South  Dakota 620 

14.  Enlargement  of  feldspar  fragment 626 

15.  Enlargement  of  hornblende  fragment 626 

16.  Clastic  quart/  penetrated  by  serpentine 643 

27 


28  ILLUSTRATIONS. 

Pagr. 

FIG.  17.  Granulation  of  feldspar  and  gradation  between  undulatory  extinction  and  granu- 
lation            674 

18.  Granulation  of  quartz  in  a  rock  in  which  the  feldspar  is  but  little  affected i>74 

19.  Liquid-filled  cavities  extending  across  several  quartz  individual*  without  change  of 

direction •  746 

20.  Diagram  showing  possible  relation  of  old  and  new  grains  of  recrystallized  rocks 752 

21.  Diagrams  illustrating  mass  deformation  of  a  rock 769 

22.  Sketch  of  oval  irregular  grains  of  calcite  with  longer  diameters  parallel 810 

23.  Graywacke  undergoing  serpentinization  along  cracks 

24.  Conglomerate  deposited  in  depression  produced  by  erosion  of  basic  dike  through 

gneiss !'-'•'» 

25.  Diagrams  illustrating  the  manner  in  which  deformation  in  the  zone  of  flowage  may 

concentrate  crustal  shortening  in  the  zone  of  fracture 928 

26.  Ideal  vertical  section  of  the  flow  of  water  entering  at  a  number  of  points  on  a  slope 

and  passing  to  a  valley  below  through  a  homogeneous  medium  interrupted  by  two 

open  vertical  channels,  one  on  the  slope  and  one  in  the  valley 1076 

27.  Ideal  section  showing  underground  circulation  in  which  no  water  anywhere  ascends 

before  issuing  at  the  surface 1080 

28.  Cross  section  of  banded  vein  near  the  London  shaft,  Mineral  Point,  Colorado 1135 

29.  Diagrammatic  section  of  Enterprise  mine,  Colorado,  and  its  blanket  pay  shoot 1208 

30.  Diagram  illustrating  mingling  of  circulations  of  two  limestones  separated  by  a  shale.  1209 

31.  Ideal  vertical  section  of  flow  of  underground  water  in  the  Galena  limestone  of  the 

upper  Mississippi  Valley 1210 

32.  Ore  deposit  in  limestone  beneath  impervious  shale,  Elkhorn  mine,  Montana 1214 


LETTER  OF  TRANSMITTAL. 


DEPARTMENT  OF  THE  INTERIOR, 
UNITED  STATES  GEOLOGICAL  SURVEY, 
SECTION  OF  PRE-CAMBRIAN  AND  METAMOKPHIC  GEOLOGY, 

Madison,  Wis.,  April  30,  1903. 

SIR:  I  transmit  herewith  the  manuscript  of  a  treatise  on  metamorphism, 
to  be  published  as  a  monograph. 

This  treatise  is  an  attempt  to  reduce  the  phenomena  of  metamorphism 
to  order  under  the  principles  of  physics  and  chemistry,  or,  more  simply, 
under  the  laws  of  energy.  The  first  nine  chapters  treat  of  metamorphism; 
the  last  three  chapters,  of  the  relations  of  metamorphism  to  stratigraphy, 
to  the  redistribution  of  the  elements,  and  to  ore  deposits. 

In  the  preparation  of  this  monograph  I  have  had  important  assistance 
from  various  sources.  The  late  Prof.  George  H.  Williams,  of  Johns 
Hopkins  University,  before  his  death  had  begun  to  accumulate  material 
and  notes  upon  metamorphism.  He  had  made  a  careful  abstract  of  the 
most  important  literature  on  the  subject;  also  a  draft,  consisting  of  about 
twenty  pages  of  manuscript,  of  a  first  chapter.  All  of  this  material 
Mrs.  Williams  turned  over  to  me.  The  summary  of  literature  prepared 
by  Dr.  Williams  has  been  of  very  great  service.  To  this  dear  friend,  the 
first  great  teacher  of  petrology  in  this  country,  I  dedicate  this  volume. 

In  the  actual  preparation  of  the  manuscript  I  have  had  the  assistance 
of  a  number  of  men,  and  of  a  considerable  number  of  advanced  students. 
In  the  earlier  work  Dr.  C.  K.  Leith  aided  me  much  by  looking  up  and 
summarizing  literature  and  by  offering  many  valuable  suggestions.  Later 
Mr.  W.  N.  Smith  continued  this  work.  To  the  discriminating  judgment  of 
Dr.  Leith  and  Mr.  Smith  I  am  greatly  indebted.  Mr.  A.  T.  Lincoln  has 
made  all  the  numerical  computations  in  reference  to  the  volume  relations  of 
original  and  secondary  minerals,  and  Mr.  R.  M.  Chapman  has  verified  Mr. 
Lincoln's  work.  While  these  are  the  men  who  have  assisted  me  most,  a 
number  of  graduate  students,  both  at  the  University  of  Wisconsin  and  at 
the  University  of  Chicago,  have  helped  in  various  ways. 

29 


30  LETTER  OF  TKANSMITTAL. 

Geologists  who  write  in  other  languages  than  English  will  have  just 
cause  for  complaint  because  of  scant  reference  to  publications  on  ineta- 
morphism  in  such  languages,  but  the  arduous  work  of  preparing  this  treatise 
lias  taxed  my  eyes  to  the  utmost  without  going  exhaustively  through 
foreign  literature. 

If  this  attempt  to  treat  one  phase  of  the  phenomena  of  geology  from 
the  point  of  view  of  energy  proves  successful,  I  shall  hope  that  it  will  lead 
to  similar  treatment  of  other  parts  of  this  great  science. 
Very  respectfully,  your  obedient  servant, 

CHARLES  RICHARD  VAN  HISE. 
Hon.  CHARLES  D.  WALCOTT, 

Director  of  United  States  Geological  Survey. 


A  TREATISE  ON  METAMORPHISM. 


By  CHARLES  RICHARD  VAN  HISE. 


CHAPTER  I. 

INTRODUCTION. 

Following  Powell,  I  shall  regard  the  earth  as  composed  of  four 
spheres — the  atmosphere,  the  hydrosphere,  the  lithosphere,  and  the  ceutro- 
spliere."  The  terms  atmosphere  and  hydrosphere  need  no  definition.  The 
term  lithosphere,  as  here  used,  will  be  confined  to  that  portion  of  the  outer 
part  of  the  earth  which  is  within  the  limits  of  observation.  How  far  below 
the  surface  observation  extends  is  somewhat  uncertain;  but  it  is  certain  that, 
in  consequence  of  deformation  and  denudation,  we  may  observe  rocks 
which  have  been  several  thousands  of  meters  below  the  surface.  Clarke 
suggests  that  the  zone  of  observation  be  defined  as  extending  to  a  depth  of 
10  miles  (Ifi  kilometers)  below  the  level  of  the  sea.6  Whatever  distance 
be  taken  as  the  limit  of  the  zone  of  observation,  it  is  certain  that  such 
distance  is  but  a  very  small  fraction  of  the  radius  of  the  earth.  All  the 
earth  below  this  fraction  will  be  considered  as  the  centrosphere ;  but  no 
hypotheses  are  advanced  in  respect  to  any  essential  difference  in  character 
between  the  material  of  the  lower  part  of  the  lithosphere  and  that  of  the 
upper  part  of  the  centrosphere. 

GENERAL  NATURE  OF  ALTERATIONS. 

The  data  of  geology  have  become  so  numerous  as  to  be  almost  unman- 
ageable. Not  many  decades  ago  it  was  possible  for  a  geologist  to  have  a 

a  Powell,  J.  W.,  Physiographic  processes:   Nat.  Geog.  Mon.,  vol.  1,  No.  1,  1895,  p.  1. 
^Clarke,  F.  W.,  The  relative  abundance  of  thechemical  elements:  Bull.  U.  S.  Geol.  Survey  No. 
78,  1891,  p.  34. 

31 


32  A  TREATISE  ON  MKTAMORPHISM. 

reasonably  full  and  satisfactory  knowledge,  not  only  of  the  known  principles 
of  geology,  l)ut  of  the  observed  phenomena  in  the  parts  of  the  world  which 
had  been  studied.  While  by  many  years  of  work  a  geologist  may  still  be 
able  to  learn  the  important  facts  concerning  various  provinces,  it  is  no 
longer  possible  for  one  man  to  have  anything  like  complete  information  as 
to  the  local  geology  of  many  parts  of  the  world.  Not  only  is  this  so,  but 
no  one  geologist  can  know  all  the  important  discovered  facts  concerning  a 
particular  branch  of  geology.  Moreover,  in  recent  years  the  accumulation 
of  facts  has  gone  on  much  faster  than  the  development  of  geological  theory. 
Nowhere  is  this  more  true  than  in  the  branch  of  geology  known  as 
petrology,  and  in  petrology  it  is  perhaps  more  true  of  the  phenomena  of  the 
alterations  of  rocks  than  of  any  other.  Scarcely  a  paper  on  petrology 
appears  that  does  not  contain  some  account  of  the  alterations  of  minerals  or 
of  rocks,  but  in  most  cases  there  is  110  serious  attempt  to  arrange  the  observed 
phenomena  in  order  tinder  recognized  principles.  Indeed,  there  is  110 
general  set  of  recognized  principles  under  which  the  phenomena  can  be 
reduced  to  order. 

Some  years  ago,  finding  myself  lost  in  the  vast  accumulation  of  data, 
I  began  to  formulate  principles  applicable  to  the  alterations  of  rocks.  The 
result  of  this  work  is  the  present  treatise,  which  is  an  attempt  to  reduce  the 
phenomena  of  metamorphism  to  order  under  the  principles  of  physics  and 
chemistry,  or,  more  simply,  under  the  laws  of  energy.  It  is  but  a  part  of 
the  larger  task  of  reducing  to  order  under  the  same  laws  the  entire  subject 
of  physical  geology. 

/  As  a  result  of  the  development  of  the  science  of  petrology,  especially 
microscopical  petrology,  it  has  been  ascertained  that  changes  are  continu- 
ally occurring  within  the  rocks  constituting  the  outer  part  of  the  earth. 
This  statement  is  equally  applicable  to  the  most  porous  rocks  at  the  surface 
of  the  earth  and  to  the  densest  rocks  as  deep  below  the  surface  as  observa- 
tion gives  exact  knowledge.  All  changes,  by  whatever  forces,  agents,  and 
processes  caused,  and  in  whatever  classes  of  rocks  occurring,  whether 
solidified  magmas,  chemical  precipitates,  organic  deposits,  or  mechanical 
deposits,  may  be  called  metamorphism. 

Metamorphism,  as  here  used,  means  any  change  in  the  constitution  of 
any  kind  of  rock. 

It  will  be  shown  that  at  any  given  time  and  place,  under  any  given 


ROCKS  ADAPTED  TO  ENVIRONMENT.  33 

set  of  conditions,  minerals  tend  to  form  which  remain  permanent  under 
those  conditions.  This  tendency  is  more  potent  with  minerals  crystallizing 
from  magmas  than  with  minerals  which  constitute  the  sedimentary  rocks 
or  with  the  secondary  minerals  which  form  by  metamorphism.  The  reason 
for  this  is  that  adjustment  to  existing  conditions  is  so  much  more  readily 
accomplished  in  fluids  than  in  solids;  but  the  tendency  to  form  minerals 
which  are  permanent  under  the  existing  conditions  controls  in  the  solid 
rocks,  although  there  is  a  great  amount  of  lag  in  the  process  of  modifica- 
tion. If  adjustment  be  reached  in  a  given  case  and  if  the  conditions 
remain  the  same,  the  minerals  formed  do  not  again  alter,  but  may  remain 
the  same  through  eons.  This  is  illustrated  by  the  meteorites,  the  minerals 
of  which  may  persist  without  change  during  the  evolutions  of  stellar  sys- 
tems. However,  when  an  important  change  of  conditions  occurs,  as  when 
a  meteor  gives  up  its  separate  existence  in  the  interstellar  spaces  and  joins 
a  planet,  as  the  earth,  readjustment  begins  at  once. 

Although  the  changes  of  conditions  upon  the  earth  are  not  so  great  as 
the  change  when  a  meteor  falls  to  the  earth,  the  range  of  conditions  upon 
the  earth  is  large  and  varied.  The  conditions  may  be  those  of  ordinary 
pressure  and  temperature  at  or  near  the  surface  of  the  earth,  or  they  may 
be  those  of  very  high  pressure  and  temperature,  such  as  exist  well  below 
the  surface  of  the  earth.  A  rock  mass  may  alternately  be  subject  to  each 
of  these  sets  of  conditions  and  to  various  intermediate  conditions.  Changes 
of  physical  conditions  result  from  surficial  transfer  of  material  by  epigene 
agents — bringing  rocks  to  the  surface  here,  burying  them  there — from 
igneous  intrusions,  from  erogenic  movement,  and  from  other  causes.  The 
changes  upon  the  earth  are  therefore  profound,  although  usually  slow. 

During  the  changes  the  rocks  are  always  modified  in  the  direction  of 
adjustment  to  the  new  conditions.  Such  modification  of  rocks  has  led  to 
the  idea  of  adaptation  to  their  environment.  As  conditions  change,  species 
of  plants  and  animals  are  so  rapidly  modified  that  at  first  sight  adaptation 
seems  almost  perfect.  Indeed,  so  sensitive  are  plants  and  animals  to  their 
environment  that  since  the  theory  of  evolution  gained  ascendancy  the  fact 
of  approximate  adaptation  is  taken  for  granted.  The  variety  and  complexity 
of  the  structures,  colors,  etc.,  of  life  forms  resulting  from  adaptation  to 
environment  is  a  constant  source  of  wonder.  Almost  daily  some  remarkable 
structure  or  form  is  described,  and  its  existence  explained  by  showing  how 
MOX  XLVII — 04 3 


34  A  TREATISE  ON  METAMORPHISM. 

it  is  advantageous  to  the  animal  under  the  conditions  in  which  it  lives. 
Since  adaptation  is  an  assumed  law,  in  those  cases  where  there  seems  to  be 
lack  of  adaptation,  as  where  some  peculiar  structure  is  present  which 
apparently  is  not  of  advantage  to  an  animal  or  plant,  it  is  believed  that  the 
facts  are  not  fully  known  or  that  the  structure  was  once  useful  and  is  a 
survival.  However,  the  very  idea  of  survival  shows  that  to  a  certain  degree 
the  development  of  plants  and  animals  lags  behind  their  changing  environ- 
ment. Upon  a  priori  grounds  it  would  be  certain  that  this  is  the  case;  and 
the  existence  of  rudimentary  organs,  such  as  the  muscles  for  moving  the 
human  ear,  which  at  one  time  may  have  had  a  use,  is  positive  evidence  of 
the  lag  of  organic  species  during  their  adaptation  to  changing  environment. 

Likewise  it  is  believed  that  minerals  constantly  tend  to  change  to 
forms  that  are  relatively  stable  under  existent  conditions.  This,  however, 
is  accomplished  by  granulation  or  recrystallization  or  some  analogous 
process,  and  is  adaptation  only  in  the  sense  that  the  old  particles  break 
up  into  smaller  particles  or  develop  into  new  mineral  particles  which 
conform  to  the  existent  conditions.  Some  minerals  are  stable  under  a 
considerable  variety  of  conditions,  and  therefore  are  less  sensitive  to 
change  than  are  others.  For  instance,  quartz  develops  directly  from  an 
igneous  rock,  and  it  also  forms  as  a  deposit  from  water.  It  persists  under 
both  quiescent  and  dynamic  conditions.  Other  minerals  require  rather 
definite  conditions  for  their  existence.  Such  are  leucite  and  olivine,  which 
abundantly  form  as  original  minerals  in  igneous  rocks  of  certain  composi- 
tion, but  which  readily  change  under  new  conditions  to  other  minerals. 
However,  no  mineral  persists  without  reference  to  its  environment,  and  so 
it  may  be  said  that  there  is  a  tendency  in  all  mineral  substances  to  form 
minerals  adjusted  to  the  conditions  under  which  they  exist.  Rocks  are 
composed  of  aggregates  of  different  minerals.  Therefore  rocks,  like 
minerals,  have  a  tendency  toward  adjustment  to  their  environment. 

Even  if  the  chemical  composition  of  a  small  mass,  say  a  cubic  milli- 
meter, remains  exactly  the  same,  the  mineral  constituents  of  the  mass 
may  greatly  change.  At  the  end  of  the  change  the  original  minerals  may 
not  be  in  the  same  proportions  as  before  and  minerals  which  did  not 
originally  exist  in  the  rock  may  have  formed.  But  the  adjustment  of  rocks 
is  not  confined  to  redistribution  of  the  elements  present  in  a  small  space. 
There  may  be  a  change  in  the  average  chemical  composition  of  rocks. 


ADAPTATION  TO  ENVIRONMENT  SLOW.  35 

Material  may  be  intruded,  or  may  be  brought  in  by  water  solutions,  or 
may  be  abstracted  by  water  solutions.  By  any  one  of  these  processes  or 
by  any  combination  of  them  a  considerable  change  in  the  chemical 
composition  of  a  rock  may  take  place. 

While  it  may  be  safely  asserted  that  all  rocks,  under  all  conditions, 
at  all  times,  are  being  adapted  to  their  environment,  the  change  in  a  rock 
goes  on  so  slowly  that  its  lag  behind  the  change  in  the  environment  may 
be  measured  by  millions  of  years.  Often  the  lag  is  so  great  that  the  con- 
ditions again  change  before  the  process  of  adjustment  has  made  much 
advancement,  and,  therefore,  before  one  set  of  changes  is  near  completion 
another  set  is  begun.  Indeed,  a  later  change  in  conditions  may  be  a 
reversal  of  an  earlier  change,  and,  therefore,  in  the  process  of  adaptation, 
work  done  in  an  early  stage  may  be  reversed  at  a  later  stage.  But  even  in 
such  a  case  it  is  clear  that  the  principle  of  adaptation  applies,  just  as  in  the 
case  of  many  plants  and  animals,  although  there  may  be,  in  fact,  little  more 
than  a  tendency  toward  adaptation  to  existent  conditions. 

Because  the  adjustment  of  rock  to  environment  is  so  slow,  in  order  that 
it  may  be  approximately  complete  it  is  necessary  that  a  rock  remain  under 
substantially  the  same  conditions  for  a  very  long  time.  This  has  happened 
in  some  regions  in  which  important  mechanical  movements  have  not 
occurred  for  a  period  or  an  era  and  the  rocks  of  which  have  remained 
buried  to  a  moderate  depth  for  most  of  the  time.  Such  were  the  conditions 
of  the  ancient  volcanics  of  certain  parts  of  the  Lake  Superior  region.  These 
have  escaped  important  mechanical  movement  since  the  beginning  of  Pale- 
ozoic time.  They  were  buried  under  Paleozoic  sediments  to  a  moderate 
depth.  Denudation  since  the  beginning  of  Cretaceous  time  brought  them 
to  the  surface.  Finally  glacial  erosion  removed  a  skin  of  weathered 
material  and  exposed  the  volcanic  rocks,  approximately  adapted  to  their 
past  environment,  that  of  the  belt  of  cementation  under  quiescent  condi- 
tions. (See  Chapter  VII,  pp.  594  et  seq.)  So  far  as  they  have  reached  the 
surface  they  are  subject  to  a  new  set  of  conditions;  and  a  new  cycle  of 
change,  begun  at  the  end  of  the  Glacial  epoch,  but  not  far  advanced,  is  in 
progress. 

In  considering  metamorphism,  the  fundamental  hypothesis  of  geology 
will  be  applied  as  in  other  branches  of  the  subject.  That  is  to  say, 
the  Huttonian  principle,  that  the  present  is  the  key  to  the  past,  is 


36  A  TREATISE  ON  METAMORPHISM. 

assumed.  Where  certain  phenomena  are  now  produced  by  certain  com- 
binations of  forces  and  agents,  and  by  these  only,  and  similar  phenom- 
ena are  found  in  the  rocks  long  since  formed,  it  is  assumed  that  the  like 
phenomena,  present  and  past,  are  due  to  essentially  the  same  combina- 
tions of  forces  and  agents.  For  instance,  if  alterations  of  a  certain 
kind  are  now  being  produced  by  a  complex  set  of.  geological  factors,  and 
by  these  only,  where  similar  alterations  are  found  in  ancient  rocks  it  is 
assumed  that  they  are  due  to  practically  the  same  combination  of  the 
forces  and  agents  of  alteration. 

But  the  above  statement  does  not  imply  that  the  changes  are  now 
taking  place  with  the  same  speed  as  that  with  which  they  occurred  in  the 
past,  as  might  have  been  held  by  Lyell;  nor  is  it  assumed  that  the  various 
forces  and  agents  have  the  same  relative  values.  Indeed,  it  is  believed  to 
be  highly  probable  that  there  have  been  changes  in  the  rate  of  alteration 
of  rocks  and  in  the  nature  and  effectiveness  of  the  factors  producing  the 
alterations. 

While  the  Huttonian  principle  is  of  service  in  the  study  of  metamor- 
phisrn,  the  alterations  of  rocks  take  place  so  slowly  that  it  does  not  have 
nearly  the  value  that  it  has  in  the  study  of  the  work  of  the  epigene  agents, 
such  as  air  and  water  and  ice;  nor  the  value  it  has  in  the  study  of  such 
hypogeue  agents  as  volcanoes  and  earthquakes. 

Many  of  the  rock  alterations  are  now  taking  place  under  conditions 
which  can  not  be  directly  observed,  but  must  be  inferred  from  the  records 
of  the  change.  This  statement  applies  to  all  changes  below  a  inile  in 
depth,  and  it  is  very  largely  applicable  to  all  but  the  mere  outer  film  of  the 
rocks,  for  most  excavations  and  cuttings  are  not  deeper  than  a  few  score  or 
a  few  hundred  feet  and  the  deepest  shafts  are  but  little  over  a  mile.  For- 
tunately it  frequently  happens  that  in  a  rock  formation  now  at  the  surface 
the  results  of  various  stages  of  change  under  deep-seated  conditions  are 
preserved,  so  that  the  character  of  the  alterations  and  the  nature  of  the 
forces  and  agents  which  have  produced  them  may  be  inferred  from  a  close 
study  of  the  different  stages  of  alteration. 

In  such  cases,  instead  of  observing  the  forces  and  agents  accomplishing 
certain  results,  and  of  reasoning  that  similar  results  produced  in  the  past 
are  due  to  these  forces  and  agents,  we  observe  the  results  at  various  stages 
of  development  and  infer  from  them  the  nature  of  the  forces  and  agents 
producing  them;  we  then  infer  that  similar  forces  and  agents  are  at  work 


METHOD  OF  REASONING.  37 

beyond  the  zone  of  observation,  accomplishing  at  the  present  time  similar 
results.  This  is  a  complete  reversal  of  the  Huttonian  method.  Hence,  in 
treating  of  metamorphism  we  must  argue  both  from  the  present  to  the  past 
and  from  the  past  to  the  present.  By  studying  the  action  of  the  forces  and 
agents  now  at  work  in  the  zone  of  observation  and  the  stages  of  alteration 

o  o 

preserved  in  the  rocks  brought  into  the  zone  of  observation  we  are  able  to 
push  the  boundaries  of  the  known  for  a  certain  distance  into  the  domain  of 
the  unknown,  and  infer  with  considerable  certainty  the  nature  of  the 
changes  which  have  taken  place  in  the  far-distant  past  and  of  those  which 
are  now  taking  place  but  which  we  can  not  directly  observe. 

It  will  be  generally  agreed  that  the  majority  of  the  altered  rocks, 
including  a  large  portion  of  the  schists  and  gneisses,  have  been  metamor- 
phosed from  aqueous  and  igneous  rocks  like  those  now  being  produced. 
This  is  in  accordance  with  the  Huttonian  principle.  But  some  may  hold 
that  the  most  ancient  of  the  schists  and  gneisses,  those  of  the  so-called 
Basement  Complex,  had  a  different  origin.  For  instance,  it  has  been  held 
by  some  that  these  ancient  rocks  are  direct  precipitates  in  a  primeval 
ocean.  On  later  pages  it  will  be  seen  that  the  most  ancient  schists  and 
gneisses  are  in  all  respects  like  those  produced  from  more  recent  rocks  by 
the  processes  of  alteration,  and  therefore  that  the  probable,  but  not  certain, 
inference  is  that  they  were  produced  from  rocks  not  fundamentally  unlike 
those  now  being  formed  by  processes  of  change  not  .radically  different  from 
those  now  at  work.  But  in  ascertaining  the  forces  and  agents  and  their 
method  of  work  in  both  the  ancient  and  the  modern  rocks,  we  must  for  the 
most  part  follow  the  reversal  of  the  Huttonian  principle — i.  e.,  argue  from 
past  results  as  to  the  nature  and  method  of  work  of  present  forces  and 
agents. 

Whatever  the  origin  of  rocks — whether  solidifications  from  magmas, 
chemical  precipitates,  organic  deposits,  or  mechanical  deposits — as  already 
noted,  they  may  be  altered  so  as  to  modify  their  structures,  so  as  to  change 
their  mineral  composition,  and  so  as  to  change  their  chemical  composition. 
In  place  of  the  original  characteristic  structures  of  the  igneous  rocks,  such 
as  flowage  structure  and  massive  structure,  and  in  place  of  the  original 
structures  of  the  sedimentary  rocks,  such  as  bedding,  there  may  be  pro- 
duced secondary  structures,  such  as  cleavage,  fissility,  joints,  slatiness, 
schistosity,  and  gneissosity.  In  place  of  the  original  textures  of  igneous 
rocks,  such  as  granolitic,  porphyritic,  ophitic,  and  poikilitic,  and  in  place 


38  A  TREATISE  ON  METAMORPHISM. 

of  the  original  textures  of  sedimentary  rocks,  such  as  granular  and  oolitic, 
there  may  be  produced  textures  characteristic  of  the  metamorphic  rocks, 
such  as  cataclastic,  parallel  orientation,  etc.  The  alteration  may  result  in 
the  change  to  minerals  all  of  which  may  wholly  differ  from  any  of  the 
original  minerals,  or  it  may  take  place  by  recrystallization  without  change 
in  mineral  character,  as  in  the  case  of  the  formation  of  marble  from  lime- 
stone. Chemical  change  may  result  in  the  addition  of  constituents,  as  in 
the  case  of  oxidation  and  hydration  of  compounds  already  existing,  or  in 
the  deposition  of  additional  material  in  the  interstices,  or  in  the  abstraction 
of  material.  Any  given  mineral  may  gain  additional  elements,  or  a  greater 
proportion  of  some  of  the  elemeiits;  it  may  lose  a  part  or  all  of  some  of  its 
elements,  or  it  may  be  wholly  replaced  by  another  mineral.  While  the 
chemical  composition  of  the  rock  may  be  greatly  affected  by  such  changes, 
in  other  cases  the  alterations  may  result  merely  in  a  redistribution  of  the 
elements  without  affecting  the  average  composition  of  the  rock,  as  in  the 
case  of  marmorization,  some  cases  of  devitrification,  various  cases  of 
metasomatism,  etc. 

After  one  set  of  changes  has  taken  place,  or  while  they  are  in  progress, 
a  change  of  physical  conditions  may  come  about  in  consequence  of  which 
a  different  set  of  changes  may  be  set  up.  Thus  rocks  may  be  partly  modi- 
fied under  mass-static  conditions  and  subsequently  modified  under  mass- 
mechanical  conditions.  They  may  be  modified  near  the  surface  of  the 
earth,  and  as  a  result  of  burial  be  later  modified  at  much  greater  depth; 
or  they  may  be  modified  at  great  depth,  and  as  a  result  of  erosion  be 
brought  near  the  surface  and  there  be  again  modified.  Therefore  one  set 
of  changes  may  be  superimposed  upon  another.  In  many  cases  it  is  cer- 
tain that  rocks  have  gone  through  several  very  complex  sets  of  modifica- 
tions. *  For  instance,  a  rock  may  be  modified  under  conditions  at  the 
surface,  afterwards  be  buried  under  other  strata  and  thus  pass  into  a  deep» 
zone,  where  it  may  be  modified  in  a  different  manner,  and  still  later,  as  a 
result  of  denudation,  be  brought  to  the  surface  and  in  the  passage  undergo 
successive  alterations  in  intermediate  belts,  and  when  it  reaches  the  surface 
once  more  be  altered  by  the  same  forces  and  agents  as  at  first.  Substan- 
tially this  history  has  been  gone  through  by  the  jaspilites  of  the  Lake 
Superior  region.  (See  pp.  831-833.)  Many  other  rocks  have  had  an 
equally  intricate  but  very  different  history. 


AGENCIES  CONCERNED  IN  ROCK  CHANGES  39 

CLASSIFICATION  OF  METAMOBPIIISM. 

The  forces  of  metamorphisin  are  chemical  energy,  gravity,  and  heat 
and  light.  The  agents  of  metamorphism  are  gases,  liquids,  and  organic 
compounds. 

A  critical  examination  of  the  published  classifications  of  metamorphism 
shows  that  the  kinds  of  metamorphism  recognized  are  based  upon  the  idea 
that  one  force  or  agent  or  process  is  dominant  in  the  production  of  a 
particular  kind  of  rock.  But  in  all  of  the  various  kinds  of  metamorphism 
ordinarily  recognized  in  classifications,  such  as  thermo-metamorphism, 
hydro-metamorphism,  chemical  metamorphism,  static  metamorphism,  pres- 
sure metamorphism,  dynamo-metamorphisni,  regional  metamorphism,  and 
contact  metamorphism,  all  of  the  forces  above  mentioned  are  required,  and 
also  the  chief  agent,  water.  There  is  no  metamorphism  of  a  rock  without 
the  presence  of  heat,  and  hence  all  metamorphism  is  partly  thermo- 
metamorphism;  there  is  no  metamorphism  without  the  presence  of  water 
solutions,  and  hence  all  metamorphism  is  partly  hydro-metamorphism;  there 
is  no  metamorphism  in  which  chemical  action  does  not  enter,  and  hence  all 
metamorphism  is  partly  chemical  metamorphism;  there  is  no  metamorphism 
without  motion,  and  hence,  in  an  exact  sense,  all  metamorphism  is  dynamic. 
In  the  alterations  of  rocks  the  forces  of  metamorphism  in  each  case  produce 
atomic,  molecular,  and  mechanical  changes."  When  it  is  realized  that  in 
all  the  varieties  of  metamorphism  mentioned  chemical  action,  heat,  and 
dynamic  action  enter  as  important  factors,  and  that  water  is  present  and 
active  wherever  metamorphism  occurs,  it  becomes  self-evident  that  the 
classifications  ordinarily  given  are  not  satisfactory.  Moreover,  the  classifi- 
cations involve  different  factors  not  belonging  to  the  same  category,  some 
being  physical,  some  chemical,  some  geological, •some  referring  to  an  agent, 
others  to  a  cause.  For  instance,  thermo-metamorphism  refers  to  heat; 
hydro-metamorphism  refers  to  the  presence  of  water;  chemical  metamor- 
phism refers  to  the  action  of  chemical  forces;  static  metamorphism  and 
pressure  metamorphism  refer  to  quiescent  conditions;  dyuamo-metamorphism 
refers  to  conditions  of  motion;  regional  metamorphism  refers  to  the  extent 

«If  this  be  true,  it  is  clear  that  a  classification  of  metamorphism  into  paramorphism,  metatrophy, 
and  metataxis,  restricting  these  terms  to  atomic,  molecular,  and  mechanical  changes,  respectively, 
as  proposed  by  A.  Irving,  is  wholly  impracticable.  Irving,  A.,  Metamorphism  of  rocks,  London. 
1389,  pp.  4-5. 


40  A  TREATISE  ON  METAMORPHISM. 

of  the  alterations;  and  contact  metamorphism  refers  to  the  contiguity  of 
an  igneous  rock. 

As  a  matter  of  fact,  all  of  these  different  kinds  of  metamorphism  are 
related  in  the  most  intricate  manner,  and  certain  metamorphic  results 
which  have  been  attributed  to  one  of  these  forces,  agents,  or  processes 
could  equally  well  be  attributed  to  another.  For  instance,  in  many  cases 
metamorphism  known  as  thermo-metamorphism  might  just  as  well  be 
called  hydro-metamorphism,  or  regional  metamorphism  be  called  dynamic 
metamorphism,  or  contact  metamorphism  be  called  thermo-metamorphism 
or  chemical  metamorphism. 

It  follows  from  the  above  that  a  satisfactory  classification  of  meta- 
morphism based  upon  chemical  forces  alone,  or  physical  forces  alone,  or 
individual  processes,  is  quite  out  of  the  question.  It  appears  to  me  that 
the  only  workable  classification  of  metamorphism  is  geological.  (See 
pp.  43-44.) 

GEOLOGICAL  FACTORS  AFFECTING  THE  ALTERATIONS  OF  ROCKS. 

The  more  important  geological  factors  affecting  the  alterations  of  rocks 
are:  Composition;  structures  and  textures;  porosity;  water  and  gaseous 
content;  climatic  and  geographic  conditions ;  ^ time ;  environment;  degree 
of  movement;  depth.  Many  physical  factors  enter  into  each  of  these 
geological  factors. 

At  present  only  general  statements  will  be  made  with  reference  to 
these  factors,  but  on  later  pages  the  effect  of  each  of  them  will  more 
clearly  appear. 

composition — In  so  far  as  rocks  are  composed  of  minerals  which  are 
permanent  under  the  existing  conditions,  or  are  composed  of  minerals 
which  may  exist  under  a  wide  variety  of  conditions,  this  is  favorable  to 
stability. 

structures  and  textures — Jn  so  far  as  there  are  coarse  structures  and  textures, 
this  is  favorable  to  permanency,  for  it  will  be  seen  that  fine  material  is 
more  readily  altered  than  coarse  material. 

porosity — Porosity  has  a  very  important  influence  upon  the  rapidity  of 
change.  In  proportion  as  rocks  are  porous  the  agents  of  alteration,  gases 
and  water,  may  enter  and  rapidly  circulate.  In  proportion  as  they  are 
dense,  the  amount  of  water  present  is  small  and  the  circulation  is  slow. 


GEOLOGICAL  FACTORS  AFFECTING  ROCK  ALTERATION.   41 

Hence  porosity  is  favorable  to  rapid  change;  density  is  favorable  to 
stability. 

water  and  gaseous  content — Jn  proportion  as  rocks  contain  water  and  gas 
they  are  readily  altered.  In  proportion  as  water  and  gas  are  absent  they 
are  stable. 

Climatic  and  geographic  conditions TllC     gp66Cl     of    alteration     of    1'OcltS   is    affected 

by  their  geographical  position.  The  alteration  of  surface  rocks  is  more 
rapid  in  tropical  than  in  arctic  regions;  it  is  more  rapid  in  humid  than  in 
arid  regions;  it  is  more  rapid  on  steep  than  on  gentle  slopes;  it  is  more 
rapid  along  coasts  than  in  the  interior.  In  short,  the  nature  of  the  altera- 
tions of  the  upper  belt  of  rocks  varies  with  every  varying  factor  of  climate 
and  geography. 

Time — Time  is  a  factor  of  the  very  highest  importance  in  metamor- 
phism.  Time  can  not  be  included  among  the  forces  or  the  agents  of  meta- 
morphism,  but  the  amount  of  metamorphism  is  a  function  of  the  time. 
Where  a  given  set  of  forces  and  agents  is  at  work  under  a  given  set  of 
conditions,  increase  of  time  increases  the  metamorphism,  but  not  in  a  direct 
ratio,  for  in  proportion  as  adjustment  to  environment  is  approached  the 
alterations  decrease  in  speed.  The  importance  of  time  in  geology  can  not 
be  too  strongly  emphasized,  for  a  comparatively  weak  force  or  agent 
working  through  a  great  length  of  time  may  accomplish  an  almost  incred- 
ible amount  of  work.  We  are  accustomed  to  judge  of  the  efficiency  of  a 
force  or  agent  by  observations  in  the  chemical  or  physical  laboratory,  but 
the  time  through  which  an  experiment  may  be  continued  in  the  laboratory 
is  an  almost  infinitely  small  fraction  of  the  time  through  which  the  forces 
and  agents  have  been  at  work  in  nature.  To  illustrate,  in  the  chemical 
laboratory  the  amount  of  crystallized  silica  which  can  be  dissolved  in  water 
and  transported  to  another  place  within  the  time  during  which  an  ordinary 
experiment  is  carried  on  is  so  small  as  to  be  immeasurable,  and  yet  it  is 
certain  that  in  nature  water  has  dissolved  and  transported  to  other  places 
enormous  quantities  of  silica.  (See  Chapter  VII,  pp.  622-623.)  This  illus- 
tration enforces  the  fact  that  the  geologist  has  very  much  more  time  at  his 
command  than  has  the  chemist  or  the  physicist.  If  the  geologist  ignores 
this  fact,  and  reasons  in  reference  to  the  potency  of  forces  and  agents  in 
metamorphism  as  a  chemist  or  physicist  would  in  the  laboratory  in  refer- 
ence to  the  same  forces  and  agents,  he  is  certain  to  fall  into  very  serious 


42  A  TREATISE  ON  METAMORPHISM. 

error.  The  importance  of  the  time  factor  lias  been  recognized  by  most 
geologists  with  respect  to  erosion  and  many  of  the  other  geological 
processes,  but  it  is  of  even  greater  importance  in  metamorphism.  Most  of 
the  metamorphic  processes  are  very  slow  indeed,  but  the  amount  of  time 
available  in  a  single  geological  period  is  great,  and  the  metamorphic  results 
are  often  stupendous. 

In  general  it  may  be  said  that  in  proportion  as  rocks  are  old  they  are 
likely  to  have  been  greatly  altered;  in  proportion  as  they  are  young  they 
are  likely  to  have  been  little  altered.  While  time  is  a  most  important 
factor  in  the  amount  of  alteration,  time  alone,  without  the  other  necessary 
conditions  for  change,  is  not  sufficient  to  insure  important  metamorphic 
results.  Further,  when  the  other  conditions  are  very  favorable  to  change, 
extensive  alteration  may  take  place  in  a  comparatively  short  time,  consid- 
ering this  factor  from  a  geological  point  of  view.  It  follows,  because  of 
variations  in  other  factors  than  time,  that  in  some  regions  very  ancient 
rocks  may  be  little  modified  and  in  other  regions  comparatively  young 
rocks  may  be  greatly  modified. 

Environment — In  many  cases  environment  may  be  important.  If  the 
rocks  surrounding  a  given  rock  be  porous,  this  condition  readily  permits  the 
entrance  of  the  agents  of  alteration — water  and  gases — and  therefore  much 
more  profound  change  may  occur  than  if  the  rock  were  surrounded  by 
comparatively  impervious  material.  This  is  illustrated  by  the  diabase  dikes 
of  the  Penokee  series  of  Michigan,  which  where  surrounded  by  the  broken 
rocks  of  the  iron-bearing  formation  are  completely  altered,  but  which 
where  surrounded  by  the  impervious  black  slates  are  comparatively 
unaltered.  A  further  very  important  factor  in  environment  is  the 
presence  of  intruded  igneous  rocks.  Igneous  rocks,  by  conduction,  may 
directly  heat  the  adjacent  rocks;  but  of  even  greater  importance  is  the 
fact  that  igneous  rocks  may  furnish  solutions  to  the  adjacent  rocks  or  heat 
the  solutions  which  percolate  through  them.  These  illustrations  show  that 
the  alteration  of  a  rock  may  be  greatly  affected  by  the  surrounding  rocks. 

Degree  of  movement — One  of  the  most  important  of  the  factors  affecting 
alterations  is  movement;  indeed,  the  factor  of  movement  is  so  important 
that  it  has  frequently  been  made  a  basis  for  a  classification  of  metamorphism. 
Changes  of  rocks  take  place  with  comparative  slowness  under  conditions 
of  quiescence  and  take  place  with  comparative  rapidity  under  conditions  of 


DEPTH  THE  MOST  IMPORTANT  GEOLOGICAL  FACTOR.    43 

movement,  Furthermore,  the  alterations  which  occur  under  dynamic  con- 
ditions are  far  more  profound  than  those  which  take  place  under  static 
conditions.  For  instance,  very  ancient  sedimentary  rocks  which  have  been 
undisturbed  by  orogenic  movements  may  be  in  almost  the  original  condition 
in  which  they  were  deposited.  On  the  other  hand,  rocks  of  comparatively 
recent  age  which  have  been  in  mountain-making  areas  and  been  deeply 
buried  may  be  profoundly  modified.  Little  metamorphosed  rocks  of  great 
age  are  illustrated  by  the  St.  Peter  sandstone  of  Wisconsin  and  the  uncon- 
solidated  Cambrian  sands  of  Russia.  Profoundly  metamorphosed  rocks 
of  comparatively  recent  age  are  illustrated  by  the  Eocene  and  Neocene 
rocks  of  the  Coast  Range  of  California  and  the  Eocene  of  the  Alps. 

Depth. — Rocks  at  or  near  the  surface  of  the  earth  are  ordinarily  under 
conditions  of  slight  pressure  and  low  temperature.  Rocks  at  some  depth 
below  the  surface  are  under  conditions  of  considerable  pressure  and 
temperature.  It  will  be  shown  that  the  alterations  of  a  given  rock  under 
these  varying  conditions  are  very  different.  Therefore  depth  is  a  matter  of 
great  consequence  in  the  consideration  of  metamorphism.  Indeed,  depth 
is  believed  to  be  the  most  important  of  the  influences  which  determine  the 
character  of  the  alterations  of  rocks.  Therefore  the  geological  factor 
which  in  this  treatise  will  serve  as  the  primary  basis  for  a  classification  of 
metamorphism  is  the  dominant  factor  of  depth.  On  this  basis  metamor- 
phism will  be  classified  into  (1)  alterations  in  the  zone  of  katamorphism 
and  (2)  alterations  in  the  zone  of  anamorphism.  The  zone  of  katamor- 
phism is  subdivided  into  (a)  the  belt  of  weathering  and  (b)  the  belt  of 
cementation.  The  zone  of  katamorphism  may  be  defined  as  the  zone  in 
which  the  alterations  of  rocks  result  in  the  production  of  simple  com- 
pounds from  more  complex  ones.  The  zone  of  anamorphism  may  be 
defined  as  the  zone  in  which  the  alterations  of  rocks  result  in  the  pro- 
duction of  complex  compound's  from  more  simple  ones.  The  belt  of 
weathering  is  the  belt  which  extends  from  the  surface  to  the  level 
of  ground  water.  The  belt  of  cementation  is  the  belt  which  extends 
from  ground-water  level  to  the  zone  of  anamorphism. 

It  is  to  be  noted  not  only  that  this  classification  is  geological,  but  that 
the  factor  is  one  which  is  universally  applicable.  Geological  factors  of 
different  kinds,  such  as  movement,  contact  action,  etc.,  are  not  introduced. 
It  is  therefore  clear  that  the  proposed  classification  follows  one  law  of  all 


44  A  TREATISE  ON  METAMORPHISM. 

good  classifications,  viz,  that  a  factor  or  factors  of  the  same  class  shall  be 
used  throughout  as  a  primary  basis.  While  the  primary  classification  of 
metamorphism  will  be  based  upon  depth,  it  is  recognized  that  there  are  no 
sharp  dividing  lines  between  the  zones  and  belts.  In  metamorphism,  as  in 
every  other  branch  of  geology  and  of  science,  there  is  complete  gradation 
between  the  phenomena  of  the  various  classes. 

However,  it  has  been  seen  that  depth  is  not  the  only  geological  factor 
of  consequence  in  metamorphism.  It  is  recognized  that  various  other 
geological  factors  enter  into  the  alteration  of  a  given  rock.  Moreover,  these 
various  factors  overlap.  In  the  discussion  of  the  zones  of  metamorphism 
the  geological  factors  of  subordinate  importance  will  be  given  proper 
consideration. 

Before  considering  the  general  alterations  in  the  zones  of  katamor- 
phism  and  anamorphism,  and  the  alterations  of  the  individual  minerals 
and  rocks  in  these  zones,  it  is  necessary  to  consider  the  forces  and  the 
agents  of  metamorphism  from  chemical  and  physical  points  of  view. 

It  should  therefore  be  recalled  that  the  forces  of  metamorphism  are 
chemical  energy,  gravity,  and  heat  and  light,  and  that  the  agents  of 
metamorphism  are  gases,  liquids,  and  organic  compounds.  The  rocks 
are  the  materials  upon  which  these  forces  and  agents  work.  The  forces 
of  metamorphism  are  considered  in  Chapter  II,  the  agents  of  metamor- 
phism in  Chapter  HI,  and  the  work  of  these  forces  and  agents  upon  the 
rocks  in  the  later  chapters. 


CHAPTER   II. 

THE  FORCES  OF  METAMORPHISM. 

As  already  seen,  the  important  forces  of  metamorphism  are  chemical 
energy,  gravity,  and  heat  and  light. 

CHEMICAL  ENERGY. 

When  different  compounds  are  brought  together  molecular  interchange 
may  occur  between  them.  As  a  result  the  compositions  of  the  compounds 
are  mutually  changed.  Such  interchange  is  chemical  action.  Chemical 
action  usually  involves  expenditure  of  chemical  energy,  which  is  one  of  the 
main  original  sources  of  energy;  but  it  will  be  seen  that  other  forms  of 
energy  may  be  transformed  into  chemical  energy,  and  chemical  action  in 
this  way  be  promoted. 

Chemical  action  may  take  place  between  gas  and  gas,  gas  and  liquid,  gas 
and  solid,  liquid  and  liquid,  liquid  and  solid,  and  solid  and  solid.  Chemical 
action,  or  molecular  interchange,  involves  movement  between  the  atoms  and 
molecules.  Chemical  action  therefore  never  takes  place  without  dynamic 
action.  So  far  as  we  know  chemical  action  never  takes  place  without  the 
presence  of  heat.  Under  the  conditions  obtaining  in  the  crust  of  the  earth 
chemical  action  is  usually  promoted  by  heat  and  by  mechanical  action.  As 
chemical  action  always  produces  a  heat  effect,  positive  or  negative,  such 
action  may  result  in  the  liberation  or  in  the  absorption  of  heat.  The  heat 
effect  may  hasten  or  retard  further  chemical  action.  In  so  far  as  chemical 
action  results  in  the  liberation  of  heat,  it  usually  hastens  further  chemical 
action,  and  therefore  promotes  metamorphism;  in  so  far  as  chemical  action 
results  in  the  absorption  of  heat,  it  usually  retards  further  chemical  action, 
and  therefore  stays  metamorphism.  It  is  shown  (pp.  170-186)  that  both 
classes  of  reactions  take  place  on  a  very  extensive  scale. 

In  consequence  of  chemical  action  material  may  be  added  to  or  sub- 
tracted from  a  given  mineral.  A  mineral  may  alter  into  two  or  more  other 
minerals  with  the  simultaneous  addition  or  subtraction  of  material.  Two 
or  more  minerals  may  unite  to  produce  a  single  mineral.  Either  of  these 

45 


46  A  TREATISE  ON  METAMORPHISM. 

changes  may  take  place  without  addition  of  material,  or  added  material 
may  be  derived  from  some  other  particle  or  particles  near  or  remote. 
Material  subtracted  from  any  given  mineral  particle  may  be  added  to 
another  mineral  particle  at  a  greater  or  less  distance.  Illustrating  the  above 
are  the  alterations  of  feldspar  into  muscovite  and  quartz,  and  of  olivine 
into  serpentine,  magnesite,  magnetite,  and  quartz.  Chemical  action  is  in 
most  cases  accomplished  through  solutions.  Therefore  its  detailed  discus- 
sion is  considered  in  connection  with  the  agents  of  metamorphism,  gaseous 
solutions,  and  aqueous  solutions.  (See  Chapter  III.) 

GRAVITY. 

Gravity  is  now  the  great  dominating  force  of  the  universe.  Indeed, 
it  is  a  main  original  source  of  energy.  Certainly  it  is  the  source  of  energy 
which  has  largely  controlled  the  development  of  the  solar  system,  including 
the  sun  and  all  the  planets  and  satellites.  The  transformations  of  gravity 
into  chemical  energy,  heat,  light,  and  other  forms  of  energy  are  important 
factors  in  the  development  of  the  solar  system,  including  the  earth.  More- 
over, gravity  still  remains  as  the  great  dominating  force  which  controls 
earth  movements,"  both  vertical  and  horizontal,  and  also  the  circulation  of 
the  water,  both  overground  and  underground.  By  earth  movements  are 
meant  all  movements  of  the  solids  or  rocks  of  the  earth  not  in  solution.  In 
this  broad  sense  the  movement  of  glaciers  is  an  earth  movement. 

The  direct  work  of  gravity  in  metamorphism  may  be  considered  under 
two  headings — mechanical  action  and  water  action. 

MECHANICAL  ACTION. 

Rocks  may  be  stressed  within  the  elastic  limit,  or  the  stress  may 
extend  beyond  the  resisting  power  of  the  material.  In  either  case  the 
rocks  are  strained.  Strain  may  occur  with  or  without  chemical  action. 
Strain  is  always  accompanied  by  some  transfer  of  energy  into  heat. 
When  the  rocks  are  strained  the  molecules  are  moved  with  reference  to 
one  another.  If  the  strain  be  within  the  elastic  limit  and  chemical  change 
does  not  take  place,  the  molecules  are  only  slightly  farther  apart  or  closer 
together,  and  when  the  stress  is  removed  they  may  return  to  their  original 

"Van  Hise,  C.  R.,  Earth  movements:  Trans.  Wisconsin  Acad.  Sci.,  Arts,  and  Letters,  vol.  11, 
1898,  pp.  512-514. 


MECHANICAL  ACTION.  47 

positions,  or  nearly  so.  If  under  the  stress  chemical  interchange  also  takes 
place  between  the  molecules,  when  the  stress  is  removed  the  body  may  still 
return  to  nearly  its  original  form.  But  if  the  strain  extends  beyond  the 
elastic  limit  the  form  of  the  body  is  notably  changed,  as  when  a  piece  of 
wrought  iron  or  steel  is  drawn  out  or  when  a  piece  of  cast  iron  is  crushed. 
Mechanical  action  may  therefore  be  considered  as  molecular  or  mass. 
Bv  molecular  mechanical  action  is  meant  differential  movements  of  the 
molecules.  By  mass  mechanical  action  is  meant  differential  movements  of 
lar<;e  mas>i-s  of  the  rocks.  Molecular  movement  also  frequently  involves 
differential  movements  of  the  atoms.  Metamorphisin  by  molecular  move- 
ment has  generally  been  called  static  metarnorphisni.  But  molecular 
mechanical  action  is  always  accompanied  in  some  degree  by  mass 
mechanical  action,  though  this  process  may  be  subordinate.  The  term 
"dvuauiic  metamorphism''  has  usually  been  restricted  to  alterations  in  con- 
nection with  mass  deformation.  But  mass  mechanical  action  is  always 
accompanied  by  molecular  mechanical  action  as  an  important  and  essential 
concomitant,  although  this  invariable  relation  has  not  always  been  recog- 
nized. Further,  as  mass  movement  becomes  important  molecular  move- 
ment, instead  of  becoming  less  important,  is  likely  to  be  of  even  greater 
consequence.  There  is  therefore  gradation  between  molecular  mechanical 
action  and  mass  dvnamic  action. 


MOLECULAR   MECHANICAL   ACTION'. 


Molecular  mechanical  action  involves  various  degrees  of  movements. 

Presumably  the  lesser  movements  are  the  cases  of  change  in  crystalline 
form  and  of  strain  within  the  elastic  limit.  In  the  change  of  a  substance 
from  one  crystalline  form  to  another — as,  for  instance,  of  aragonite  to  cal- 
cite — the  movement  of  the  molecules  may  not  involve  more  than  a  rear- 
rangement of  those  which  are  adjacent.  In  the  case  of  substances  strained 
within  the  elastic  limit,  the  molecules  are  simply  pressed  slightly  closer 
together  or  pulled  slightly  farther  apart,  and  yet  these  very  slight  adjust- 
ments may  have  a  profound  effect  upon  the  physical  properties  of  the 
materials.  For  instance,  amorphous  glass  when  strained  but  slightly  and 
well  within  its  elastic  limit  becomes  an  anisotropic  substance.  Leucite 
crystallizes  in  the  isometric  system  at  high  temperatures.  As  the  mineral 
cools  it  passes  at  once  into  an  anisotropic  form.  The  transformation  from 


48  A  TREATISE  ON  METAMORPHISM. 

one  to  the  other  may  be  seen  by  alternately  heating  and  cooling  this 
mineral  under  the  microscope.  In  the  foregoing  cases,  while  we  can  not 
doubt  that  movement  occurs,  the  readjustment  is  molecular,  and  it  is  there- 
fore beyond  the  power  of  the  microscope  to  determine  its  character. 

It  might  at  first  be  supposed  that  such  slight  movements  as  are  involved 
in  strains  within  the  elastic  limit  are  unimportant,  but  it  is  to  be  remembered 
that  strains  of  this  kind  not  only  affect  every  mineral  particle,  but  displace 
the  individual  molecules  with  reference  to  one  another,  so  that  the  strained 
masses  are  affected  throughout.  While,  therefore,  it  requires  polarized 
light  to  detect  the  strained  condition  in  minerals,  it  is  certain  that  the  effect 
is  pervasive.  It  will  be  seen  (pp.  95-98)  that  such  state  of  strain  is  of 
fundamental  importance  in  the  matter  of  solution  and  deposition  through 
the  agency  of  solutions. 

In  a  second  class  of  movements  there  is  molecular  interchange  between 
substances  by  which  the  compounds  are  modified  in  composition.  Such 
interchanges  involve  chemical  action.  The  motions  which  occur  during 
chemical  changes  in  solids  are  commonly  for  such  short  distances  that  the 
naked  eye  does  not  discover  the  relations  of  the  original  and  secondary 
minerals.  Such  movements  are  microscopic.  Chemical  interchange  may 
be  mainly  accomplished  by  chemical  forces  and  the  movement  be  an 
incident  of  this  process.  On  the  other  hand,  mechanical  action  may  be  the 
inciting  cause  which  leads  to  chemical  action.  And,  finally,  the  purely 
chemical  and  mechanical  forces  may  interact,  each  promoting  the  other. 
The  more  important  chemical  reactions  resulting  from  mechanical  action 
are  accomplished  through  the  agency  of  solutions,  and  hence  are  treated 
in  Chapter  III.  But  Prof.  Walther  Spring"  has  shown  that  chemical 
changes  may  be  induced  by  mechanical  action  alone,  without  the  presence 
of  solutions.  For  instance,  when  barium  carbonate  and  solid  sodium 
sulphate  were  mixed  in  equal  molecular  proportions  and  subjected  to  a 
pressure  of  6,000  atmospheres  a  change  took  place  by  which  80  per  cent 
of  the  barium  carbonate  and  sodium  sulphate  were  changed  to  barium 
sulphate  and  sodium  carbonate,  respectively;  and  conversely,  when  barium 
sulphate  and  sodium  carbonate  were  mixed  together  in  equal  molecular 
proportions  and  subjected  to  a  like  pressure  about  20  per  cent  was  changed 

« Professor  Spring  on  the  physics  and  chemistry  of  solids,  review  by  C.  F.  Tolman,  jr.:   Jour. 
Geol.,  vol.  6,  1898,  p.  323. 


MASS  MECHANICAL  ACTION.  49 

to  barium  carbonate  and  sodium  sulphate."  In  all  such  changes  the 
fundamental  principle  controlling  is  that  reactions  shall  take  place  which 
result  in  smaller  volumes.  Spring6  found  that  in  the  case  of  dry  reactions 
induced  by  mechanical  action  time  is  a  very  important  factor,  the  reactions 
taking  place  much  more  slowly  than  when  compounds  are  moist  and  water 
is  an  intermediate  agent. 

MASS   MECHANICAL   ACTION. 

Mass  mechanical  action  (a)  may  permanently  strain  the  rocks  without 
openings,  (b)  may  strain  the  rocks  with  rupture  and  openings,  and  (c)  may 
close  the  openings  in  rocks  and  produce  welding. 

permanent  strain  without  openings. — In  order  that  permanent  strain  beyond 
the  elastic  limit  without  openings  may  take  place  in  the  rocks  it  is  nec- 
essary that  deformation  shall  occur  while  the  rocks  are  under  a  sufficient 
pressure  in  all  directions  to  hold  the  molecules  so  close  together  that  the 
molecular  attraction  is  effective.  This  will  be  true  only  where  the  pressure 
is  greater  in  all  directions  than  the  crushing  strength  of  the  rocks.  It  is 
well  illustrated  by  Adams  and  Nicolsou's  experiment  on  the  deformation 
of  marble  while  under  pressure  in  all  directions/  The  molecules  were 
held  close  to  one  another,  and  the  deformed  marble  retained  considerable 
strength. 

Later  we  shall  see  that  the  process  of  readjustment  may  be  mechanical 
or  chemical  or  partly  each.  When  the  process  is  mechanical  the  mineral 
particles  are  usually  granulated — that  is,  finely  fractured.  When  the 
process  is  chemical  the  particles  are  recrystallized.  Also  the  process  of 
readjustment  may  be  accomplished  by  any  combination  of  granulation  and 
recrystallization  (See  pp.  737-748.)  Under  natural  conditions,  in  order 
that  the  pressure  in  all  directions  shall  be  greater  "than  the  crushing  strength 
of  a  rock,  it  is  necessary  that  it  be  in  the  zone  of  flowage  for  that  rock. 

permanent  strain  with  openings. — When  the  rocks  are  strained  beyond  the  elastic 
limit  and  the  pressure  is  not  greater  in  all  directions  than  the  crushing 
strength  of  the  rocks,  rupture  and  openings  are  produced.  The  ruptures 
may  be  regular  or  irregular.  The  regular  ruptures  may  be  of  great  extent 

oNernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
p.  390. 

6  Spring,  op.  cit.,  p.  322. 

«  Adams,  F.  D.,  and  Nicolson,  J.  T.,  An  experimental  investigation  into  the  flow  of  marble: 
Philos.  Trans.  Royal  Soc.  London,  ser.  A,  vol.  195,  1901,  pp.  363-401. 

>ION   XLVII — 04 -i 


50  A  TREATISE  ON  METAMORPHISM. 

and  wide  apart,  as  in  the  case  of  faults;  or  of  moderate  extent  and  width, 
as  in  the  case  of  joints  and  bedding  partings;  or  close  together,  as  in  the 
case  of  fissility.  The  irregular  ruptures  may  be  continuous  and  the  open- 
ings wide,  as  in  the  case  of  the  coarse  breccias;  or  discontinuous  and  the 
openings  small;  or  so  minute  as  to  affect  the  individual  particles,  and  thus 
grade  into  deformations  without  openings  or  granulation. 

Under  natural  conditions,  in  order  that  the  pressure  shall  not  exceed 
the  crushing  strength  of  a  rock  in  all  directions,  it  is  necessary  that  it  shall 
be  in  the  zone  of  fracture  for  that  rock. 

Permanent  strain  with  closing  of  openings  and  welding. Mechanical     action      may     cloS6 

openings  in  rocks  and  weld  the  separated  parts.  In  this  case  there  is  a 
diminution  of  volume  due  to  bringing  the  particles  closer  to  one  another. 
In  order  that  welding  shall  take  place  there  must  be  sufficient  pressure  in 
all  directions  to  bring  the  particles  so  close  together  that  the  molecular 
attractions  are  effective,  or  the  pressure  in  all  directions  must  be  greater 
than  the  crushing  strength  of  the  rock. 

Of  course,  the  pressure  required  to  satisfy  the  above  conditions  tor 
welding  depends  very  greatly  upon  the  character  of  the  material.  Moder- 
ate pressure  may  be  sufficient  to  weld  material  composed  of  small  and 
weak  particles.  For  instance,  moderate  pressure  of  clay  may  bring  many 
of  the  minute  particles  of  kaolin  so  close  to  one  another  as  to  place  them 
within  the  limits  of  effective  molecular  attraction.  When  the  clay  is  dried 
the  mass  becomes  harder.  This  hardening  is  doubtless  due  in  part  to  the 
precipitation  of  the  dissolved  material  contained  by  the  water  and  the  conse- 
quent cementation  of  the  particles,  as  explained  on  pages  617-621.  In  pro- 
portion as  the  particles  are  coarse,  strong,  and  large,  and  have  relatively 
few  points  of  contact,  the  pressure  necessary  to  produce  welding  increases. 
To  produce  deformation  with  welding  of  the  separated  large  particles  of 
the  strong  minerals  considerable  pressure  is  necessary. 

WATER  ACTION. 

The  •  movement  of  water  under  the  force  of  gravity  is  of  the  utmost 
importance  in  metamorphism.  It  is,  indeed,  the  great  agent  of  transporta- 
tion of  material  both  overground  and  underground,  and  is  the  dominating 
agent  through  which  metamorphism  is  accomplished.  Its  work  is  fully 
considered  in  Chapter  III,  on  "The  agents  of  metamorphism." 


FORCES  OF  METAMORPHISM.  51 

HEAT  AND  LIGHT. 

Heat  and  light  are  form's  of  energy  of  the  first  importance.  It  has 
already  been  noted  that  their  ultimate  source  is  largely  gravity.  Heat 
is  always  present  as  a  factor  in  metamorphism,  for  nowhere  upon  the  surface 
of  the  earth  nor  within  the  earth  is  the  temperature  absolute  zero.  Other 
things  being  equal,  the  higher  the  temperature  the  more  rapidly  do  alterations 
of  rocks  take  place.  Light  also  affects  all  parts  of  the  earth  at  the  surface. 
In  metamorphism  heat  and  light  should  be  considered  from  two  points  of 
view — (1)  sources  of  heat  and  light,  and  (2)  effect  of  heat  and  light  upon 
the  alterations  of  rocks. 

SOURCES  OF  HEAT  AND   LIGHT. 

Heat  and  light  agential  in  the  alteration  of  rocks  are  derived  (a)  from  the 
sun,  (b)  from  deep  within  the  earth  by  conduction  or  by  convection  through 
water  or  magma,  (c)  from  mechanical  action,  and  (d)  from  chemical  action. 
The  heat  from  all  these  sources  is  important;  light,  however,  is  derived 
chiefly  from  the  sun,  that  from  the  other  three  sources  being  of  little 
consequence. 

THE   SUN   AS   A    SOURCE    OF    HEAT   AND   LIGHT. 

The  heat  and  light  of  the  sun  are  forces  of  the  first  order  of  magnitude 
in  the  alterations  of  rocks.  The  effect  of  these  forces  needs  to  be  considered 
in  four  cycles — the  cycle  of  the  solar  system,  that  of  the  seasons,  that  of 
the  cyclone,  and  that  of  the  day. 

The  solar-system  cycle  is  the  most  important.  This  cycle  involves 
two  factors — the  absolute  temperature  and  change  in  temperature. 

As  to  the  absolute  temperature,  were  it  not  for  the  heat  and  light  of 
the  sun  it  is  certain  that  the  temperature  of  the  surface  of  the  earth  would 
not  greatly  exceed  that  of  the  interstellar  spaces.  Probably  it  would  be 
—200°  C.,  or  even  lower.  At  the  present  time  the  temperature  of  the 
surface  of  the  earth  averages  10°  C.  (283°  C.  absolute)  or  more.  Therefore 
the  temperature  of  all  the  upper  zone  of  the  earth  is  200°  C.,  or  more, 
greater  than  it  would  be  without  the  heat  from  the  sun.  Were  it  not  for 
this  heat  the  water  in  the  outer  zone  of  the  earth  would  be  congealed,  and 
the  atomic  and  molecular  energy  would  be  greatly  diminished.  As  a 
comparatively  slight  increase  of  temperature  over  that  prevalent  at  the 
surface  of  the  earth  increases  greatly  the  speed  of  alteration  of  rocks,  it  is 


52  A  TREATISE  ON  MKTAMORPHISM. 

to  be  presumed  that  under  such  low  temperatures  changes  in  rocks  would 
be  so  slow  as  to  be  negligible. 

How  deep  below  the  surface  of  the  earth  the  heat  of  the  sun  produces 
an  effect  can  not  be  accurately  determined.  It  is  highly  probable  that  it 
has  an  important  effect  to  a  depth  of  thousands  of  meters,  probably  beyond 
the  limits  of  the  zone  of  observation.  If  the  sun  were  not  furnishing  heat 
to  the  earth,  and  the  increment  of  increase  in  temperature  were  the  same 
as  at  present  (1°  C.  for  30  meters),  and  the  temperature  at  the  surface  were 
200°  C.  lower  it  would  be  necessary  to  penetrate  to  a  depth  of  6,000  meters 
to  reach  a  temperature  as  high  as  that  at  the  surface  under  the  present 
conditions.  Below  6,000  meters  the  temperature  would  increase  approxi- 
mately as  it  does  now  from  the  surface  downward.  However,  it  is  not  to 
be  supposed  that  the  effects  of  metamorphism  would  be  the  same  as  those 
in  the  outer  6,000  meters  at  the  present  time,  for  the  conditions  of  pressure 
would  be  very  different.  (See  Chapter  I,  p.  43,  and  Chapter  IV,  pp.  159- 
160.)  The  assumption  that  the  increment  of  temperature  would  remain 
the  same  were  not  the  sun  giving  heat  to  the  earth  is  only  approximately 
true;  but  when  it  is  remembered  that  6,000  meters  is  an  exceedingly  small 
fraction  of  the  earth's  radius,  it  seems  probable  that  the  increment  of 
increase  of  heat  with  depth  in  the  outer  part  of  the  crust  of  the  earth 
would  not  be  greatly  different,  even  if  the  sun  had  long  ceased  to  be  a 
source  of  heat;  but  if  it  were  not  for  the  heat  of  the  sun,  the  temperature 
of  that  part  of  the  lithosphere  directly  under  observation  would  be  so  low 
that  all  chemical  clianges  would  be  very  slow,  if  indeed  they  were  not 
inappreciable. 

The  absolute  temperature  at  the  surface  is  also  dependent  upon 
latitude.  The  average  temperature  at  the  warmest  tropical  regions  is  about 
300°  C.  absolute,  or,  stated  in  the  ordinary  scale,  27°  C.;  the  average 
temperature  of  the  coldest  polar  region  where  observations  have  been  made 
(latitude  81°  44')  is  252.9°  absolute,  or,  in  the  ordinary  scale,  —20.1°  C." 
At  intermediate  latitudes  there  are  all  gradations  between  these  extremes. 
At  any  place  the  temperature  may  be  presumed  to  increase  with  depth  from 
these  surface  temperatures  at  the  rate  of  1°  C.  per  30  meters. 

It  would  be  fruitless  to  attempt  a  discussion  of  the  changes  of  the 
temperature  of  the  outei'  part  of  the  earth  due  to  the  solar  cycle.  So  far 

"Hann,  Julius,  Handbuch  <ler  Klimatologie,  J.  Engelhorn,  Stuttgart,  1883,  p.  733. 


CHANGES  IN  TEMPERATURE.  53 

as  rock  alterations  now  taking  place  are  concerned,  the  sun  may  be  regarded 
as  furnishing  to  the  earth  a  uniform  amount  of  heat. 

The  seasonal  changes  of  temperature  are  very  important,  at  the  surface 
rano-ino-  from  30°  C.  or  less  to  as  much  as  80°  C.  However,  the  depth  to 

O         C  * 

which  the  seasonal  change  produces  an  effect  is  not  great,  probably  about 
15  meters. 

The  cyclonic  changes  of  temperature  may  be  very  great,  ranging  from 
a  few  degrees  to  about  70°,  but  the  depth  to  which  these  changes  extend  is 
slight,  probably  less  than  3  meters. 

The  diurnal  changes  in  temperature  are  scarcely  less  than  the  cyclonic, 
ranging  from  0  to  50°  C.  or  more;  but  the  depth  to  which  the  diurnal 
changes  extend  is  insignificant,  probably  but  a  fraction  of  a  meter. 

From  the  foregoing  it  is  plain  that  the  heat  and  light  derived  from  the 
sun  are  of  very  great  direct  importance  in  the  chemical  and  mechanical 
changes  that  rocks  undergo.  It  will  also  be  seen  that  the  various  changes 
of  temperature  as  well  as  the  absolute  temperatures  are  of  great  consequence. 
Moreover,  the  heat  and  light  of  the  sun  exert  a  very  important  indirect 
influence  upon  metamorphism  by  reason  of  their  being  the  sole  source 
of  the  energy  which  produces  plants  and  animals,  and  these  agents  will  be 
seen  to  have  a  far-reaching  effect  upon  the  alterations  of  rocks.  The  effects 
of  the  heat  and  light  derived  from  the  sun  are  fully  considered  in  Chapters 
VI,  VII,  and  VIII,  on  "The  belt  of  weathering,"  "The  belt  of  cementation," 
and  "The  zone  of  anamorphism." 

HEAT  DERIVED  FROM  WITHIN  THE  EARTH  BY  CONDUCTION  OR  CONVECTION  THROUGH 

WATER  OR  MAGMA. 

The  amount  of  heat  derived  by  the  crust  of  the  earth  from  the  interior 
depends  upon  the  conductivity  of  the  various  rocks  and  upon  the  convec- 
tional  movements  of  magma  and  water. 

The  heat  conductivity  of  the  majority  of  rocks  is  between  0.4  and  0.6, 
silver  having  a  conductivity  of  100.  It  is  apparent  that  the  conductivity 
of  rocks  is  very  low  as  compared  with  that  of  the  metals,  but  it  can  not  be 
doubted  that  there  is  a  steady  but  slow  flow  of  heat  by  conduction  from  the 
interior  of  the  earth  to  the  zone  of  observation. 

The  amount  of  heat  derived  by  the  crust  of  the  earth  from  intrusions 
of  igneous  rocks  is  very  great.  So  far  as  this  heat  passes  into  the  adjacent 


54  A  TREATISE  ON  METAMORPHISM. 

rocks  by  conduction,  the  coefficients  are  the  same  as  in  the  transfer  of  heat 
from  the  interior  of  the  earth.  The  transfer  of  the  heat  of  magma  to  adjacent 
rocks  is  probably  largely  accomplished  by  convection.  The  magmas  fre- 
quently furnish  heated  solutions.  Ordinary  circulating  waters  approach  or 
come  in  contact  with  the  igneous  rocks;  they  thus  became  heated.  The 
heated  waters  move  through  the  rocks  controlled  by  the  laws  of  under- 
ground circulating  waters  (see  pp.  146-153),  and  give  up  a  part  of  their  heat 
to  the  surrounding  rocks.  The  important  metamorphosing  effects  of  the 
great  igneous  masses  through  water  convection  may  extend  several  miles. 
Contact  metamorphisni  is  sometimes  restricted  to  the  very  marked  effects 
due  to  high  temperature  immediately  adjacent  to  the  igneous  rock.  How- 
ever, the  alterations  thus  produced  by  high  temperature  as  the  result  of 
direct  conduction  are  probably  small,  compared  with  the  widespread  effects 
resulting  from  the  dispersal  of  heat  and  material  by  means  of  underground 
waters. 

MECHANICAL    ACTION    AS   A    SOURCE    OF   HEAT. 

It  is  a  well-known  principle  that  when  work  is  done  involving  strain 
of  solids  within  the  elastic  limit,  or  subdivision  of  solids,  or  differential 
movement  between  solids  in  contact,  the  energy  is  partly  transformed  to 
heat.  Hence  strain  within  the  elastic  limit,  subdivision  of  the  rocks,  and 
differential  movement  between  rock  masses  and  particles  and  within  the 
particles  raise  the  temperature  of  the  rocks,  and  this  greatly  increases  the 
speed  and  extent  of  the  chemical  reactions.  Heat  developed  by  mechanical 
action  is  therefore  an  important  factor  in  the  metamorphism  of  rocks. 
Indeed,  the  resultant  metamorphic  products  are  very  different  under  con- 
ditions of  movement  and  under  conditions  of  quiescence;  but  heat  is  only 
one  of  the  factors  entering  into  the  differences.  (See  pp.  685—707.) 

CHEMICAL   ACTION    AS   A    SOURCE   OF   HEAT. 

Chemical  action  always  produces  a  positive  or  negative  heat  effect, 
and  thus  promotes  or  retards  metamorphism. 

EFFECTS  OF  HEAT  AND  LIGHT  ON  ALTERATIONS  OF  ROCKS. 

The  relations  between  metamorphism  and  heat  and  light  may  be  gener- 
ally stated  as  follows:  The  kinetic  energy  of  the  molecules  of  substances, 
whether  in  the  form  of  gas,  liquid,  or  solid,  is  increased  by  heat  and  light 


EFFECTS  OF  HEAT  AND  LIGHT.  55 

The  speed  of  metamorphism  is  therefore  largely  dependent  upon  the  amount 
of  heat  and  light  present,  especially  the  former. 

In  rock  alteration  heat  and  light  produce  direct  effects  and  indirect 
effects. 

DIRECT    KFFECTS   OF   HEAT   AND   LIGHT. 

The  more  important  direct  effects  may  be  either  mechanical  or 
chemical. 

Mechanical  effects. — The  mechanical  effects  are  desiccation,  baking,  and 
fusion.  At  the  surface  of  the  earth  the  heat  of  the  sun  frequently  results 
in  evaporating  the  moisture  and  desiccating  the  rocks;  an  attendant  result 
is  induration.  This  process  is  especially  important  in  the  clay  sediments, 
and  occurs  to  the  greatest  extent  in  the  hot  and  arid  regions,  although 
desiccation  is  not  unimportant  in  the  colder  regions.  The  details  of  the 
process  especially  concern  the  belt  of  weathering  and  are  treated  in  the 
chapter  on  that  subject.  (See  Chapter  VI,  pp.  541-550.)  Where  igneous 
rocks  as  a  consequence  of  volcanism  are  brought  into  contact  with  other 
rocks  the  latter  may  be  baked  for  a  longer  or  shorter  distance  from  the 
igneous  rocks.  The  process  of  baking  as  here  used  is  restricted  to  modifi- 
cations similar  to  those  which  take  place  in  the  baking  of  bricks;  that  is,  to 
effects  which  are  mainly  due  directly  to  the  heat.  This  process  is  restricted 
to  the  belt  above  the  level  of  underground  water — the  belt  of  weathering — 
and  is  therefore  treated  in  detail  in  the  chapter  on  that  subject.  (See 
Chapter  VI,  pp.  488-494.)  Below  the  belt  of  weathering  the  rocks  are 
saturated  with  water  and  the  heat  effects  are  mainly  produced  through  that 
agent.  Even  in  the  belt  of  weathering  the  baking  effect  is  not  wholly  due 
to  heat,  but  is  partly  accomplished  through  the  agency  of  the  contained 
water,  precisely  as  is  the  transformation  of  clay  to  brick  by  burning;  for  all 
rocks  under  natural  conditions  contain  gas  and  water,  and  usually  consid- 
erable quantities.  During  the  baking  process  the  original  molecules  are 
brought  nearer  together,  but  there  are  also  important  chemical  changes. 

Where  the  masses  of  the  igneous  rocks  are  very  great,  and  especially 
where  adjacent  rocks  are  included  in  masses  of  igneous  rocks,  the  rocks 
may  be  softened  by  the  heat  or  even  absorbed  by  the  magma.  Where 
the  rocks  are  softened  they  are  likely  to  be  very  greatly  changed,  perhaps 
recrystallized.  Where  they  are  absorbed  by  the  magma  they  are  lost  as 
original  rocks  and  become  a  part  of  the  magma  by  which  they  are  absorbed. 


56  A  TREATISE  ON  METAMORFHISM. 

When  the  modified  magma  crystallizes  it  takes  the  form  of  an  ordinary 
igneous  rock,  and  may  show  no  evidence  of  the  fact  that  previously 
solidified  rocks  have  contributed  material. 

chemical  effects. — In  proportion  as  the  temperature  is  high  chemical  reactions 
are  likely  to  take  place  between  solids.  This  is  illustrated  by  the  case- 
hardening  of  iron.  When  soft  iron  is  placed  in  contact  with  pulverized 
charcoal  and  the  temperature  is  raised  to  a  red  heat,  but  not  to  the  point 
of  fusion,  some  of  the  carbon  unites  with  the  iron,  transforming  the  outer 
part  of  it  into  steel.  Thus  it  is  casehardened.  Just  how  the  union  takes 
place  between  the  iron  and  the  carbon  is  uncertain.  It  is  supposed  to  be  due  to 
the  direct  union  of  the  solids,  but  we  can  not  be  quite  sure  that  the  result 
is  not  accomplished  through  the  agency  of  a  gas.  The  carbon  may  be  partly 
oxidized,  and  thus  be  transformed  to  the  gas  carbon  monoxide.  This  may 
penetrate  the  iron,  which  may  reduce  the  carbon  monoxide  to  carbon  again. 
The  reduced  carbon  may  at  the  instant  of  reduction  unite  with  the  iron, 
forming  the  carbide,  or  steel.  While  it  is  certain  that  high  temperature  is 
favorable  to  the  mutual  chemical  reactions  of  solids,  when  the  temperature 
becomes  so  high  as  to  transform  the  solids  to  liquids  the  chemical  reactions 
are  those  of  liquids  rather  than  those  of  solids. 


INDIRECT   EFFECTS   OF   HEAT   AND   LIGHT. 


The  indirect  effects  of  heat  and  light  are  accomplished  through  the 
agents  of  metamorphism — gases,  water,  and  organic  forms.  The  move- 
ments of  the  atmosphere  and  hydrosphere  are  the  conjoint  effect  of  heat 
and  light  and  gravity.  It  has  already  been  noted  that  the  movements  of 
these  bodies  are  the  agents  which  do  the  main  work  of  epigene  transfer 
of  material.  Not  only  do  gas  and  water  act  as  agents  of  transfer,  but  they 
act  as  agents  for  chemical  changes.  It  has  already  been  seen  that  chem- 
ical action  may  be  a  direct  result  of  heat.  However  this  may  be,  it  is 
certain  that  by  far  the  more  important,  indeed  the  dominant,  effects  which 
heat  and  light  have  upon  chemical  reactions  are  accomplished  through  the 
agency  of  gases  and  water  and  organic  forms.  Of  the  forces  heat  and 
light,  the  former  is  the  important  one  in  the  reactions  accomplished  through 
the  agency  of  gases  and  water  solutions ;  but  light  is  very  important  in  the 
production  of  organic  agencies.  The  indirect  effects  of  heat  and  light  and 
.all  other  conjoint  forces  are  considered  in  connection  with  the  agents  of 
alteration  in  Chapter  III. 


EFFECTS  OF  HEAT  AND  LIGHT.  57 

GENERAL,   STATEMENTS. 

From  the  foregoing  it  is  apparent  that  the  effects  of  chemical  energy, 
gravity,  and  heat  and  light  are  not  independent  of  one  another;  on  the 
contrary,  they  are  most  intricately  interlocked.  To  a  considerable  degree 
any  one  of  the  forms  of  energy  may  be  transformed  into  the  others.  Con- 
sequently the  action  of  one  almost  always  produces  an  effect  upon  the 
action  of  the  others.  Moreover,  one  almost  never  acts  without  the  action 
of  the  others.  Frequently  all  of  the  forces  of  metamorphism  are  important 
simultaneous  factors  in  the  results;  again,  one  or  two  of  the  forces  may  be 
prominent,  or  even  dominant,  the  others  -playing  a  subordinate  part.  But, 
in  every  transformation  of  metamorphism,  if  all  the  energy  factors  of  the 
entire  system  affected  be  taken  into  account,  some  of  the  energy  is  changed 
into  the  lowest  form  of  energy,  heat,  and  at  least  a  portion  of  this  heat  is 
dissipated." 

«Daniell,  Alfred,  A  text-book  of  the  principles  of  physics,  3d  ed.,  Macmillan  Co.,  New  York, 
1895,  p.  51. 


CHAPTER  III. 

THE  AGENTS  OF  METAMORPHISM. 

GENERAL  STATEMENT. 

The  agents  through  which  the  alterations  of  rocks  take  place  are 
gaseous  and  liquid  solutions  and  organisms.  Solutions  are  the  special 
subject  of  this  chapter.  Organisms  are  influential  only  in  the  belt  ot 
weathering,  and  their  action  is  therefore  considered  in  connection  with  that 
belt.  (See  Chapter  VI.) 

The  circulation  and  work  of  solutions  involve  a  consideration  of  the 
circulation  and  work  of  the  gases  of  the  earth,  of  which  the  atmosphere  is 
the  dominant  portion,  and  a  consideration  of  the  circulation  and  work  of  the 
water  of  the  earth,  of  which  the  ocean  is  the  dominant  portion.  While  the 
circulation  and  work  of  the  atmosphere  and  of  overground  water  may  from 
a  purely  theoretical  point  of  view  be  considered  as  a  part  of  a  treatise  on 
metamorphism,  the  work  of  these  epigene  agents  is  the  subject  of  that 
division  of  geology  which  has  been  named  physiography,  and  as  the  work 
of  the  atmosphere  and  overground  water  is  so  fully  dealt  with  in  connection 
with  that  subject,  this  branch  of  metamorphism  will  not  be  discussed  here 
at  all.  Hut  the  circulation  and  work  of  underground  gas  and  water  solu- 
tions are  of  fundamental  importance  in  metamorphism  and  must  be  some- 
what fully  considered. 

Gas  and  water  below  the  surface  in  the  openings  of  the  rocks  will  be 
called  ground  gas  and  ground  water,  to  discriminate  them  from  gas  and 
water  above  the  lithosphere. 

Solutions  "are  homogeneous  mixtures  which  can  not  be  separated  into 
their  constituent  parts  by  mechanical  means.""  The  properties  of  solutions 
vary  continuously  and  regularly  with  the  concentration.6  Under  the 

"Ostwald,  W.,  Solutions,  translated  by  M.  M.  Pattison  Muir;  Longmans,  Green  &  Co.,  London, 
1891,  p.  1. 

^Cameron,  F.  K.,  Application  of  the  theory  of  solutions  to  the  study  of  soils:  Rept.  No.  64,  Field 
Operations  of  the  Division  of  Soils,  1899,  U.  S.  Dept.  of  Agric.,  1900,  pp.  142-143. 

58 


CHARACTER  OF  THE  SOLUTIONS.  59 

definition,  solutions  may  be  made  by  mingling  gases  and  gases,  gases  and 
liquids,  gases  and  solids,  liquids  and  liquids,  liquids  and  solids,  and  solids 
and  solids.  The  solutions  resulting  from  these  various  combinations  may 
be  gases,  liquids,  or  solids,  or  partly  two  or  all.  Gaseous  solutions  may  be 
formed  by  the  mingling  of  gases  and  gases,  of  gases  and  liquids,  and 
of  gases  and  solids."  Liquid  solutions  may  be  formed  by  the  mingling  of 
gases  and  gases,  of  gases  and  liquids,  of  liquids  and  liquids,  of  solids  and 
liquids,  and  of  gases,  liquids,  and  solids.  Solid  solutions  may  be  formed 
by  the  mingling  of  gases  and  solids,  of  liquids  and  solids,  of  solids  and 
solids,  and  of  gases,  liquids,  and  solids.  But  however  complex  the  origin 
and  however  numerous  the  components,  the  compounds  with  which  the 
geologist  has  to  deal  are  gases,  liquids,  and  solids.  The  two  common 
combinations  which  he  has  to  consider  are  gaseous  solutions  and  solids, 
and  liquid  solutions  and  solids.  The  liquid  solutions  are  universally 
aqueous.  The  solids  are  the  rocks.  The  combinations  gaseous  solutions 
and  solids,  and  aqueous  solutions  and  solids  will  be  treated  under  Parts  I  and 

II  of  this  chapter. 

PART  I.    GASEOUS   SOI/UTIONS. 

Since  the  geological  work  of  gases  and  vapors  can  not  be  practically 
discriminated,  the  term  gas  is  here  used  to  cover  both  gases  and  vapors. 

The  gases  which  are  important  in  rock  alteration  are  oxygen  (O2), 
sulphur  (S8  to  S2),  water  gas  (H2O),  ammonia  (NH3),  carbon  dioxide  (CO2), 
sulphurous  oxide  (SO2),  boric  acid  (H3BO3),  hydrochloric  acid  (HC1),  and 
hydrofluoric  acid  (HF). 

Never  is  one  of  these  chemical  compounds  at  work  alone  upon  the 
rocks;  at  the  place  of  action  there  are  always  solutions  of  several  gases. 
Mineralizers  in  rocks,  according  to  the  original  definition,  are  substances 
which  act  in  the  gaseous  condition;6  but  it  will  be  seen  (pp.  490-494) 
that  the  term  has  been  practically  restricted  to  peculiar  gases  under  special 
circumstances.  Notwithstanding  the  definition  of  the  term,  the  action  of 
water  solutions  containing  certain  compounds,  which  if  alone  would  be 
gaseous,  has  been  spoken  of  as  due  to  mineralizers.  The  term  miner- 
alizers,  if  it  is  to  serve  any  useful  purpose,  should  be  definitely  restricted 

aDaniell,  Alfred,  A  text-book  of  the  principles  of  physics.  3d  ed.,  Macmillan  Co.,  New  York, 
1895,  p.  330. 

6  Expression  "Agents  mineralisateurs "  first  used  by  Elie  de  Beaumont  and  defined  by  H.  Ste.- 
Claire  Deville:  Comptes  rendus  des  Stances  de  I'Acad&nie  des  Sciences,  vol.  52,  1861,  pp.  920,  1264. 


60  A  TREATISE  ON  METAMORPHISM. 

to  the  action  of  some  particular  compound  or  compounds,  or  else  to  some 
form  of  compound,  such  as  gases. 

Gaseous  solutions  require  consideration  from  two  points  of  view — the 
chemical  and  physical  principles  controlling  the  action  of  gases  and  the 
geological  work  of  gases. 

SECTION  i.   CHEMICAL  AND   PHYSICAL  PRINCIPLES  CONTROLLING  THE  ACTION 

OF  GASES. 

The  chemical  and  physical  principles  controlling  the  work  of  gases 
may  be  considered  under  (1)  the  gases  present,  (2)  the  pressure,  and  (3) 
the  temperature. 

Gases  present. — The  law  of  greatest  importance  controlling  the  chemical 
action  of  gaseous  solutions  is:  The  properties  of  a  homogeneous  mixture  or 
solution  of  various  gases  are  the  sum  of  the  properties  of  the  constituents  of 
the  mixture.  To  illustrate,  when  carbon  dioxide  (C02)  and  oxygen  (02)  are 
mixed  the  properties  and  activities  of  each  are  the  same  as  if  the  same 
quantity  of  each  were  free  from  the  other  and  occupied  the  same  space. 
Therefore,  in  the  belt  of  weathering,  where  gases  are  active,  the  carbon 
dioxide  and  oxygen  are  both  doing  their  work,  the  one  that  of  carbonation 
(see  pp.  473-480),  the  other  that  of  oxidation  (see  pp.  461-473),  as  if  the 
other  were  not  present.  It  is  clear,  therefore,  that  the  properties  of  gaseous 
mixtures  are  additive. 

Slight  deviations  from  this  law  have  been  noted  under  certain  condi- 
tions, but  these  mainly  concern  the  exact  physics  of  the  gaseous  solutions 
rather  than  their  geological  work,  and  hence  are  not  here  considered.  In 
applying  the  law,  however,  we  must  be  sure  that  the  gases  do  not  unite 
chemically  and  produce  a  new  compound.  To  illustrate,  while  the  law  is 
certainly  applicable  to  the  case  mentioned,  that  of  a  mixture  of  carbon 
dioxide  and  oxygen,  it  is  not  certain  that  this  is  the  case  when  water  gas 
(H2O)  and  carbon  dioxide  or  sulphurous  oxide  (S02)  are  mixed,  for  these 
compounds  may  unite  with  water  gas,  producing  carbonic  acid  (H2C03) 
and  sulphurous  acid  (H2SO3)  gases.  Certainly  the  law  will  not  apply  to  a 
mixture  of  the  gases  ammonia  (NH3)  and  water,  for  these  gases  will  largely 
unite  and  produce  ammonium  hydroxide  (NH4OH),  which  may  exist  in  the 
form  of  gas.  In  case  a  gas  be  formed  by  the  union  of  two  or  more  gases, 


PRESSURE  AI\7D  TEMPERATURE  OF  THE  GASES.  61 

the  law  controlling-  the  action  of  gases  is  applicable  to  the  new  compound, 
and  to  the  other  gases  with  which  it  is  mingled  but  does  not  unite 
chemically. 

As  to  the  relative  importance  of  the  gases,  it  might  at  first  be  thought 
that  the  strong  acids,  such  as  hydrochloric  and  hydrofluoric,  are  of  greater 
consequence  than  the  much  less  active  compounds,  carbon  dioxide  and 
oxygen;  but  it  should  be  remembered  that  carbon  dioxide  and  oxygen  are 
everywhere  at  work  upon  the  surface  of  the  earth,  whereas  the  presence 
of  the  strongly  active  compounds  in  more  than  minute  quantities  is  excep- 
tional. It  therefore  follows  that  the  action  of  the  universally  present 
weaker  agents,  such  as  carbon  dioxide  and  oxygen,  is  of  immeasurably 
greater  geological  importance  than  the  action  of  the  stronger  but  much  less 
abundant  gases. 

The  pressure. — Increase  of  pressure  increases  the  chemical  activity  of  a  gas. 
This  law  follows  from  the  fact  that  the  number  of  molecules  which  act  upon 
a  given  space  is  directly  as  the  pressure.  The  varying  atmospheric  pressure 
may  be  taken  as  illustrating  this  principle.  When  the  pressure  increases, 
say,  by  .05,  this  means  that  1.05  times  as  many  molecules  of  gas  are  actively 
at  work  upon  a  given  area  as  before. 

One  of  the  best  illustrations  of  the  increased  activity  of  gases  in  accom- 
plishing chemical  work  when  under  pressure  is  that  of  carbon  dioxide.  As 
shown  in  another  place  (pp.  175-176),  carbon  dioxide  is  capable  of  decom- 
posing many  silicates  at  ordinary  temperatures;  but  Struve  and  Mueller0 
have  shown  that  when  carbon  dioxide  is  under  pressure  its  effect  in  decom- 
posing silicates  is  very  much  greater  than  under  ordinary  conditions.  This 
is  in  accordance  with  the  law  of  mass  action.  In  proportion  as  the  pressure 
increases  the  number  of  active  molecules  increases,  and  therefore  the 
geological  work  increases  in  proportion. 

The  temperature. — The  activity  of  gases  increases  with  increase  of  temper- 
ature. In  proportion  as  the  temperature  is  high,  the  kinetic  molar  energy 
of  the  molecules  of  gases  is  great.  The  absolute  temperature  of  a  perfect 
gas  is  believed  to  be  a  direct  measure  of  its  kinetic  molar  energy.  By 
molar  kinetic  energy  is  meant  the  energy  of  translation  of  the  molecule  of 
a  gas,  and  not  the  vibratory  or  rotary  motions  of  the  molecules  themselves. 

"Mueller,  Richard,  Untersuchungen  tiber  die  Eimvirkung  des  kohlensiiurehaltigen  Wassers  auf 
einige  Mineralien  und  Gesteine:    Tschermaks  mineral.  Mittheil.,  vol.  7,  1877,  p.  47. 


62  A  TREATISE  ON  METAMORPHISM. 

The  kinetic  energy  of  a  moving  body  is  the  product  of  one-half  of  its  mass 
into  the  square  of  its  velocity.  When  a  gas  is  very  dense  its  molecules 
are  closely  crowded,  and  on  account  of  the  molecular  attraction  there  is 
an  appreciable  decrease  in  the  theoretical  pressure,  which  is  a  measure  of 
the  kinetic  molar  energy.  Since  the  kinetic  energy  of  the  gaseous  molec- 
ular projectiles  increases  as  the  squares  of  the  velocities,  this  may  explain 
why  a  slight  increase  of  temperature  often  greatly  increases  the  chemical 
reactions  of  the  gases  in  contact  with  the  solids  of  the  earth's  crust,  for 
the  likelihood  of  a  chemical  union  depends,  among  other  things,  upon  the 
energy  with  which  the  particles  of  a  gas  come  in  contact  with  the  minerals 
of  the  rocks. 

SECTION   2.     GEOLOGICAL  WORK  OF  GASES. 

The  observable  geological  work  of  gases  is  mainly  above  the  level  of 
ground  water,  or  in  the  belt  of  weathering.  In  the  belt  of  cementation, 
below  the  level  of  underground  water,  the  rocks  are  saturated  with  water 
solutions.  Gaseous  substances,  if  present,  would  be  in  solution  in  water, 
and  their  action  would  therefore  fall  under  water  solutions,  treated  on  later 
pages. 

In  the  belt  of  weathering  oxygen  and  carbon  dioxide  are  immeasura- 
bly the  most  important  of  the  mineralizers,  because  they  are  present  in  the 
interstices  of  the  rocks  in  this  belt  throughout  the  laud  areas.  However, 
in  volcanic  districts  any  or  all  of  the  geologically  important  gases  may  be 
present  and  have  a  very  marked  metamorphosing  effect  upon  the  rocks. 
But  of  these  gases  that  of  water  is  of  vastly  the  greatest  consequence. 
The  consideration  in  detail  of  the  effects  of  these  various  mineralizers  and 
of  their  action  in  conjunction  with  other  agents  properly  falls  in  Chapter  VI 
on  "The  belt  of  weathering." 

In  the  deep-seated  zone  of  anamorphism  water  itself  is  mainly  above 
its  critical  temperature  (see  pp.  659-661),  and  is  therefore  in  the  form  of  a 
gas.  On  account  of  the  great  pressure  the  gases  are  dense.  Under  these 
conditions  most  or  all  of  the  substances  held  in  solution  would  also  be  in 
the  form  of  gases.  The  active  substances  would  be  solutions  of  gases  in 
gases.  One  would  expect  that  the  action  of  water  gas  holding  in  solution 
other  gases  under  such  conditions  of  pressure  and  temperature  would  be 
different  from  the  action  of  highly  heated  water,  in  that  its  viscosity  would 


ACTION  OF  THE  GASES.  63 

be  very  small.  It  would  therefore  have  a  greater  penetrating  power  than 
water,  and  would  be  more  highly  energetic  in  its  action.  Under  these 
conditions  the  minutest  spaces  would  be  somewhat  readily  traversed.  The 
rocks  of  the  deep  zone  in  which  action  of  this  kind  has  taken  place  can 
reach  the  surface  only  by  passing  through  the  zone  in  which  water  is  in  the 
liquid  form.  Therefore  the  effects  which  were  produced  by  the  mineralizers 
in  the  deepest  zone  will  have  been  modified  by  the  action  of  water  solu- 
tions during  the  long  time  the  rocks  were  in  the  belt  of  cementation.  The 
details  of  the  effect  of  water  gases  in  the  zone  of  rock  flowage  will  be  con- 
sidered in  Chapter  VIII,  on  "The  zone  of  anamorphism." 

All  of  the  gases  may  act  in  either  of  two  ways:  (1)  By  their  presence 
they  may  influence  crystallization  or  recrystallization  without  entering  into 
combination.  (2)  They  may  enter  into  the  combinations  forming  oxides, 
hydroxides,  carbonates,  sulphates,  etc.  The  first  of  these  actions  is  spoken  of 
as  that  of  crystallizers,  and  the  second  as  that  of  mineralizers.  In  the  meta- 
morphic  rocks  it  is  ordinarily  difficult  to  prove  the  past  action  of  gases,  not 
in  water  solutions.  Occasionally  the  materials  of  volcanic  cones  have  been 
rendered  porous  and  the  rocks  altered  in  consequence  of  the  action  of 
gaseous  exhalations.  In  such  cases  the  gases  usually  have  united  to  some 
extent  with  the  materials  through  which  they  have  passed,  and  in  this  way 
furnish  evidence  of  their  past  action. 

PART  II.     AQUEOUS  SOLUTIONS  AISTD  SOLIDS. 

GENERAL    CONSIDERATIONS. 

The  one  liquid  through  which  the  greater  part  of  the  alterations  of 
rocks  occur  is  water  solution.  Indeed,  this  is  so-  profoundly  true  that  the 
water  of  the  earth  has  been  compared  with  the  blood  of  an  organism.  And  it 
is  certainly  true  that  the  transformations  of  tissues  by  the  blood  are  scarcely 
more  far-reaching  than  those  of  the  lithosphere  by  the  agency  of  water.  It 
has  been  determined  by  laboratory  experiments  that  pure  water  at  ordinary 
temperatures  is  capable  of  dissolving  all  compounds  to  some  extent.  Cor- 
responding with  this  fact,  analyses  of  ground  waters  show  that  they  contain 
in  solution  all  of  the  elements  which  occur  in  nature.  The  solutions  may 
vary  from  very  dilute  to  rather  strong.  So  far  as  the  gases  are  dissolved  in 
water,  their  action  is  to  be  treated  under  water  solutions,  not  under  gases. 


64  A  TREATISE  ON  METAMORFHISM. 

In  the  belt  of  weathering,  above  the  free  surface  of  ground  water, 
gaseous  solutions  and  liquid  solutions  work  together.  In  this  belt  the  rocks 
are  not  ordinarily  saturated  with  water,  but  on  the  average  contain  a  con- 
siderable amount  of  water  held  by  adhesion  between  the  liquid  and  the  solid 
mineral  particles.  It  is  believed  that  in  this  belt  the  gases  act  upon  the 
rocks  chiefly  through  water  solutions.  As  evidence  of  this  is  the  small 
amount  of  decomposition  of  the  disintegrated  rocks  in  arid  regions.  (See 
pp.  496-498.)  It  therefore  appears  that  the  dominant  agents  of  alterations 
in  the  belt  of  weathering  are  aqueous  solutions. 

In  the  belt  of  cementation  below  the  free  surface  of  ground  water  the 
rocks  are  practically  saturated,  and  in  this  belt  aqueous  solutions  are  the 
chief  agents  of  alterations. 

Water  solutions  are  also  a  chief  agent  in  the  transportation  of  material 
from  one  place  to  another. 

At  this  point  it  is  necessary  to  understand  that  the  places  of  interaction 
of  aqueous  solutions  and  solids  are  the  contacts  between  the  two.  It  will 
be  seen  later  that,  on  account  of  the  molecular  attraction  between  water  and 
rock,  a  thin  filni  of  water  adheres  to  the  solid  particles  with  which  it  is  in 
contact.  This  film  is  not  in  active  circulation,  yet  it  is  the  part  of  the 
agent,  water,  which  is  immediately  concerned  in  the  transfer  of  mineral 
material  from  the  rocks  to  the  solutions  and  from  the  solutions  to  the  rocks. 

The  contact  film  may  take  material  of  the  rock  into  solution.  From 
this  film  the  materials  taken  into  solution  migrate  to  other  parts  of  the 
solution.  Probably  the  migration  from  the  contact  film  to  the  free  water  is 
largely  by  diffusion  (see  pp.  82-83) ;  but,  once  beyond  the  contact  film,  the 
migration  is  largely  accomplished  by  convectional  movements.  Material 
may  be  supplied  to  the  contact  film  by  migration  of  material  from  the  free 
parts  of  the  solution.  From  the  contact  film  material  may  be  deposited  in 
the  rocks. 

In  this  connection  it  is  interesting  to  note  that  in  the  portions  of  the 
solutions  near  the  contact  with  solids  "there  is  often  a  concentration  of 
the  dissolved  material.  This  phenomenon  has  been  called  adsorption."" 
The  phenomena  of  adsorption  seem  to  show  with  great  clearness,  not  only 
that  the  contact  film  is  the  active  agent  in  transfer  between  the  free  solu- 

«  Cameron,  Frank  K.,  Application  of  the  theory  of  solutions  to  the  study  of  soils:  Report  No.  64, 
Field  Operations  of  Division  of  Soils,  1899,  U.  S.  Dept.  of  Agric.,  1900,  p.  142. 


ACTION  OF  AQUEOUS  SOLUTIONS.  65 

tions  and  the  solids,  but  that  in  this  film  the  migration  of  the  dissolved 
material  is  to  some  extent  stayed  by  the  molecular  attraction  of  the 
crystals. 

Aqueous  solutions  as  a  geological  agent  require  consideration  from  two 
points  of  view — the  chemical  and  physical  principles  controlling  the  action 
of  ground  water,  and  the  circulation  and  geological  work  of  ground  water. 
These  are  treated  in  the  following  sections  I  and  II,  respectively: 

SECTION    i.    CHEMICAL  AND  PHYSICAL  PRINCIPLES  CONTROLLING  THE  ACTION 

OF   GROUND   WATER. 

The  work  of  ground  water,  like  any  other  work,  requires  the  expendi- 
ture of  energy.  The  energy  by  which  the  water  accomplishes  its  work  is 
derived  from  chemical  action,  heat,  and  mechanical  action. 

In  order  to  comprehend  the  processes  of  alteration  of  rocks  it  will  be 
necessary  to  summarize  the  important  conclusions  of  physical  chemistry  as 
to  solutions  and  chemical  reactions.  The  principles  here  contained  are 
mainly  taken  from  the  works  of  Ostwald  and  Nernst. 

Chemical  action  will  be  considered  under  the  headings,  "Principles of 
solutions  applicable  to  ground  waters,"  and  "Principles  of  chemical  reactions 
applicable  to  ground  waters." 

PRINCIPLES   OF   SOLUTIONS   APPLICABLE    TO    GROUND    WATERS. 

While  the  consideration  of  the  principles  of  solution  logically  falls 
under  general  chemical  action,  and,  perhaps,  ought  to  be  treated  as  a  special 
case  under  the  general  treatment  of  chemical  reactions,  it  seems  advisable, 
because  the  subject  of  solutions  is  somewhat  simple  as  compared  with  the 
interactions  of  complex  chemical  compounds,  to  take  up  this  subject  first, 
after  which  the  general  laws  controlling  chemical  reactions  will  be  given. 

The  water  of  rocks,  whether  at  ordinary  temperatures  and  pressures  or 
at  higher  temperatures  and  pressures,  may  take  any  of  the  substances 
with  which  it  comes  in  contact  into  solution;  it  may  deposit  substances 
from  solution;  it  may  combine  with  substances  forming  hydroxides,  as  in  the 
case  of  many  of  the  zeolites  and  limonite;  it  may  part  with  its  hydrogen  in 
exchange  for  bases,  thus  at  the  same  time  changing  the  composition  of  the 
rock  and  taking  the  bases  replaced  into  solution.  This  is  illustrated  by  the 
alteration  of  enstatite  to  talc.  (See  Chapter  y,  p.  268.)  There  may  be 

MON   XLVII--04 5 


66  A  TREATISE  ON  METAMORPHISM. 

reactions  as  a  result  of  different  substances  being-  taken  into  solution  at 
different  times;  there  may  be  reactions  as  a  result  of  different  solutions 
conning  together,  and  thus  mingling;  there  may  be  reactions  between 
substances  in  solution  and  the  solid  material  with  which  the  water  is  in 
contact;  there  may  be  reactions  as  a  result  of  changing  temperature  and 
pressure.  All  these  changes  are  in  the  nature  of  chemical  action.  There- 
fore by  chemical  action  through  solutions  is  meant  the  taking  of  material 
into  solution,  the  deposition  of  material  from  solution,  the  interchange 
between  materials  in  solutions,  the  interchange  between  materials  in  solu- 
tions and  adjacent  solids,  and,  finally,  the  interchange  of  the  adjacent  solid 
particles,  for  such  an  interchange  is  usually  accomplished  through  the 
medium  of  a  separating  film  of  water.  In  this  case  the  apparently  simple 
reaction  between  solids  is  really  accomplished  by  transfers  through  sepa- 
rating solutions.  In  all  these  interchanges  the  materials  pass  through  a 
stage  of  solution. 

Salts  are  combinations  of  the  metals  and  the  acid  radicals.  Thus 
Na,S04  is  a  combination  of  Na2  and  S04,  and  KC1O3  of  K  and  C103. 
Faraday  called  these  constituents  ions.  This  term  will  be  used  as  defined 
by  Faraday  without  any  implication  that  a  compound  in  solution  separates 
into  its  constituent  ions  or  is  dissociated. 

According  to  many  chemists"  salts  in  various  solutions  are  at  least 
partly  separated  into  their  ions.  Such  supposed  separation  has  been  called 
electrolytic  dissociation.  If  electrolytic  dissociation  takes  place  to  a  consid- 
erable extent,  the  properties  of  the  compounds  are  practically  the  sum  of 
the  properties  of  their  separated  ions.  In  its  power  of  dissociation  of 
dissolved  salts  water  is  held  to  exceed  all  other  solvents.  Water  itself  is 
held  to  be  slightly  dissociated,  or  the  H2O  separates  into  the  ions  OH  and 
H.  According  to  the  theory  of  dissociation  the  presence  of  free  ions  in 
water  solutions  is  therefore  universal.  By  the  advocates  of  the  theory  it 
is  held  that  it  is  by  the  interaction  of  these  free  ions  that  chemical 
interchanges  are  accomplished.  But  dissociation  is  held  to  be  very  imper- 
fect in  strong  solutions,  relatively  far  advanced  in  dilute  solutions,  and  in 
very  dilute  solutions  nearly  or  quite  complete.  As  the  greater  portion  of 

"Nernst,  W.,  Theoretical  chemistry,  trans,  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
p.  307.  Ostwald,  W.,  Outlines  of  general  chemistry,  trans,  by  James  Walker,  Macmillan  &  Co.,  Lon- 
don, 1895,  pp.  266-290. 


FORM  OF  SILICA  IN  SOLUTIONS.     .    .  67 

underground  solutions  are  very  dilute,  at  least  where  somewhat  free  circu- 
lation is  the  rule,  if  the  theory  of  dissociation  be  true  we  may  suppose 
that  the  salts  held  in  solution  are  largely  separated  into  their  ions.  While 
the  theory  of  dissociation  and  the  explanation  of  chemical  reactions  by 
interchange  of  free  ions  (see  pp.  84-85)  have  a  strong  foothold  in  theoretical 
chemistry,  they  have  never  gained  universal  support;  and  recently  the 
theory  has  been  strongly  attacked  by  Kahlenberg,  who  not  only  holds 
that  the  theory  is  unnecessary  to  explain  chemical  reaction,  but  brings 
together  many  facts  which  appear  to  controvert  it."  He  has  shown,  more- 
over, that  instantaneous  chemical  changes  take  place  in  solutions  that  are 
the  best  of  insulators.6 

Until  recently  it  has  not  been  known  how  the  most  important  of  the 
geological  compounds,  the  silicates,  behave  when  dissolved.  However, 
Kahlenberg  and  Lincoln c  have  shown  that  when  dilute  solutions  of  sili- 
cates are  made  the  silica  exists  in  such  solutions  in  the  form  of  colloidal 
silicic  acid.  To  illustrate:  If  a  sufficiently  dilute  solution  of  sodium 
silicate  be  made,  but  much  more  concentrated  than  ordinarily  occurs  in 
underground  waters,  the  compound  breaks  up  into  NaOH  and  colloidal 
silicic  acid.  From  this  fact  it  would  not  be  supposed  that  the  silicic  acid  is 
a  chemically  active  compound,  and  it  is  not  active  near  the  surface  of  the 
earth  at  ordinary  temperatures  and  pressures;  but  on  subsequent  pages  it 
will  be  seen  that  at  considerable  depth,  where  the  pressure  and  temperature 
are  much  above  the  normal,  silicic  acid  is  a  most  active  compound. 

Before  the  ionic  theory  of  solutions  gained  recognition  it  was  cus- 
tomary in  the  published  analyses  of  underground  waters  to  suppose  that 
the  bases  and  acids  of  the  dissolved  materials  are  united  in  a  definite  way. 
For  instance,  chlorine  was  ordiuarilv  considered  as  united  with  the  potas- 
sium, sodium,  or  calcium.  The  sulphuric  oxide  radical  S04  was  supposed 
to  be  united  with  the  oxides  of  potassium,  magnesium,  calcium,  and  sodium. 
The  carbon  dioxide  radical  CO3  was  supposed  to  be  united  with  the  oxides 
of  iron,  magnesium,  sodium,  and  calcium.  The  aluminum  and  silica  were 

«  Kahlenberg,  L.,  The  theory  of  electrolytic  dissociation  as  viewed  in  the  light  of  facts  recently 
ascertained:  Bull.  Univ.  of  Wisconsin  No.  47,  1901,  pp.  299-351;  also  Jour.  Phys.  Chem.,  vol.  5,  1901, 
pp.  339-392. 

6 Kahlenl>erg,  L.,  Instantaneous  chemical  reactions  and  the  theory  of  electrolytic  dissociation: 
Jour.  Phys.  Chem.,  vol.  6,  1902,  p.  1. 

c Kahlenberg,  L.,  and  Lincoln,  A.  T.,  Solutions  of  silicates  of  the  alkalies:  Jour.  Phys.  Chem., 
vol.  2,  1898,  pp.  88-90. 


68  A  TREATISE  ON  METAMORPHISM. 

usually  regarded  as  oxides,  although  in  some  cases  the  aluminum  was 
treated  as  united  with  the  chlorine."  However,  results  of  recent  analyses 
have  ordinarily  been  given  on  the  basis  of  ions.6 

.  In  a  solution,  under  the  law  of  mass  action,  each  of  the  bases  is  to  be 
considered  as  divided  between  all  acids,  and  under  the  theory  of  disso- 
ciation there  are  also  present  in  the  solutions  the  free  ions  of  both  the 
bases  and  the  acids.  For  example,  suppose  a  strong  underground  water 
solution  to  contain  three  bases  and  three  acid  radicals;  as,  for  instance,  the 
bases  sodium,  calcium,  and  magnesium,  and  the  radicals  of  carbonic, 
sulphuric,  and  hydrochloric  acid;  then  the  following  nine  compounds  are 
present,  Na2CO3,  Na,S04>  NaCl,  CaCO3,  CaSO4,  CaCl2,  MgC03,  MgSO4, 
MgCl2,  and  also  the  six  free  ions,  Na,  Ca,  Mg,  C03,  S04,  and  01,  making 
altogether  fifteen  separate  combinations  of  the  elements.  However,  under 
the  theory  of  dissociation,  if  the  solutions  be  so  weak  that  the  substances  in 
solution  are  wholly  ionized  the  nine  compounds  first  mentioned  will  not  be 
present.  If  the  dissociation  theory  be  rejected,  under  the  law  of  mass 
action  in  all  cases  all  of  the  nine  compounds  will  be  present,  but  not  the 
free  ions. 

Under  the  principles  of  solutions  it  is  necessary  to  consider  the  cases 
of  (1)  the  solution  of  gases  in  ground  waters,  (2)  the  solution  of  solids  in 
ground  waters,  and  (3)  diffusion. 

SOLUTIOX  OF  C1ASES  IX  OROUJID  WATERS. 

The  quantity  of  gases  which  can  be  dissolved  in  underground  water 
depends  upon  the  gases  present,  the  pressure,  the  temperature,  and  the 
solids  in  solution. 

oases  present — A.11  the  natural  gases  may  be  dissolved  in  water  or  may 
unite  with  water.  In  the  latter  case  the  resultant  compounds  are  dissolved. 
In  both  cases  solutions  are  formed. 

Since  below  the  level  of  the  free  surface  of  underground  water  it  is 
clear  that  the  gases  enter  into  solution  either  by  absorption  or  by  combina- 
tion, it  follows  that  the  more  far-reaching  effects  of  these  substances  in 
metamorphism  are  not  as  gases,  but  as  aqueous  solutions.  The  gases  are 

a  Peale,  A.  C.,  Lists  and  analyses  of  the  mineral  springs  of  the  United  States:  Bull.  U.  S.  Geol. 
Survey  No.  32,  1886,  pp.  43,  115,  133. 

''Clarke,  F.  W.,  and  Hillebrand,  W.  F.,  Analyses  of  rocks  and  analytical  methods,  U.  8.  Geol. 
Survey,  1880-1896:  Bull.  U.  S.  Geol.  Survey  No.  148,  1897.  Clarke,  F.  W.,  Analyses  of  rocks, 
laboratory  of  the  U.  8.  Geol.  Survey,  1880-1899:  Bull.  U.  S.  Geol.  Survey  No.  168,  1900. 


GASES  IN  SOLUTIONS.  69 

therefore  important  factors  in  the  c  "lien  of  ground  waters,  but  they  are  of 
course  only  a  small  portion  of  the  substances  which  ground  waters  carry. 
The  more  important  of  these  gases  which  pass  into  ground  waters  are: 
Oxygen  (O2),  carbon  dioxide  (CO2),  hydrosulphuric  acid  (H2S),  sulphur- 
ous oxide  (SO2),  hydrochloric  acid  (HC1),  hydrofluoric  acid  (HF),  boric 
acid  (H3BO3),  and  ammonia  (NH3).  Sulphur  and  boric  acid  as  gases  occur 
mainly  in  connection  with  volcanic  action.  If  the  above-mentioned  or 
other  gases  unite  with  the  water  the  laws  below  given  as  to  solubility  do 
not  hold;  thus  carbon  dioxide  unites  with  water,  forming  carbonic  acid 
(C02  +  H20  =  H2CO3);  sulphurous  oxide  unites  with  water,  producing  sul- 
phurous acid  (SO2+H20  — H2SO3);  ammonia  unites  with  water,  producing 
ammonium  hydrate  (NH3  +  H20=NH4OH).  In  some  of  these  cases,  for 
instance,  that  of  ammonia  and  sulphurous  oxide,  the  water  may  unite  with 
many  times  its  volume  of  the  gas,  with  increase  of  volume;  thus  water  at 
0°  C.  and  atmospheric  pressure  absorbs  1,050  volumes  of  ammonia  as  a 
result  of  the  union  of  the  two.  What  portion  of  C02  contained  in  ground 
water  remains  as  CO2  in  solution,  and  what  part  unites  with  water,  forming 
carbonic  acid,  is  uncertain,  but  it  is  definitely  known  that  much  of  the  CO2 
contained  in  the  ground  water  is  in  the  form  of  the  so-called  bicarbonates — 
for  instance,  such  salts  as  Na2C03+H2CO3  or  2NaHCO3 — and  therefore  is 
united  with  the  water. 

When  new  compounds  are  formed  by  the  union  of  the  gases  with  the 
liquids,  the  substances  held  in  solution  are  the  new  compounds.  When 
these  new  compounds  are  gases  the  laws  below  given  concerning  the  solu- 
tion of  gases  in  liquids  apply  only  to  the  new  compound,  not  to  the  original 
gas.  Where  the  compound  is  a  solid — as,  for  instance,  a  bicarbonate — the 
laws  for  the  solution  of  gases  in  water  do  not  apply,  but  such  compounds 
are  held  under  the  laws  controlling  the  solution  of  solids  in  liquids.  (See 
pp.  72-82.) 

In  some  cases  in  nature  a  part  of  a  gas  may  unite  with  a  substance  in 
solution  and  make  a  new  compound  and  a  part  may  unite  with  water  and 
be  dissolved  in  this  form.  If  both  the  compounds  be  gases  the  laws  for  the 
solution  of  gases  in  liquids  hold.  If  the  new  compound  formed  be  a  solid 
salt  the  laws  for  the  solution  of  solids  in  liquids  apply  to  it,  and  the  laws 
for  the  solution  of  gases  in  liquids  apply  to  the  uncombined  gas.  This  case 
is  illustrated  by  carbon  dioxide,  already  mentioned. 


70  A  TREATISE  ON  METAMORPHISM. 

The  pre.sure. — "The  quantity  of  a  gas  dissolved  by  a  specified  quantity  of 
a  liquid  is  proportional  to  the  pressure  of  the  gas."0  This  statement  is  true 
of  each  gas  without  reference  to  whether  a  gas  be  alone  or  mixed  with  other 
gases.  Thus  the  solubility  of  each  of  a  number  of  mixed  gases  is  controlled 
by  the  pressure  exerted  by  that  gas,  not  by  the  total  pressure  exerted  by 
the  mixture.  It  is  therefore  clear  that  under  natural  conditions  the  press- 
ure of  that  part  of  any  gas  which  is  in  the  atmosphere  and  the  pressure  of 
that  part  which  is  held  in  solution  in  the  water  immediately  adjacent  are 
the  same  when  the  two  are  in  equilibrium,  and  the  water  is  therefore  just 
saturated. 

So  far  as  ground  waters  are  concerned,  there  are  two  cases;  first,  the 
waters  of  the  belt  of  weathering,  or  those  to  the  level  of  ground  water; 
and  second,  those  below  the  level  of  ground  water,  or  the  belt  of  satura- 
tion. In  the  belt  of  weathering  the  pressure  is  atmospheric.  Changes  of 
pressure  are  barometric.  In  so  far  as  the  atmospheric  pressure  varies — and 
this  is  by  fractions  up  to  one-fifteenth — the  solubility  of  the  natural  gases 
in  the  water  of  the  belt  of  weathering  also  varies  directly  as  the  pressure  of 
each  of  the  gases  varies,  without  reference  to  the  pressure  and  solubility  of 
the  other  gases. 

In  the  belt  of  saturation,  just  at  the  level  of  ground  water,  the  amount 
of  gases  held  in  solution  is  proportional  to  atmospheric  pressure;  but 
at  greater  depths  higher  degrees  of  concentration  of  gases  are  possible, 
although  it  might  at  first  be  thought  that  the  atmospheric  pressure  or  vapor 
pressure  at  the  free  surface  of  the  water  would  determine  the  concentration 
of  the  solution.  The  pressure  which  really  is  determinative  as  to  the 
amount  of  gas  which  may  be  held  in  solution  is  that  of  a  column  of  water 
extending  to  the  free  surface,  plus  the  atmospheric  pressure.  Since, 
however,  water  is  so  much  heavier  than  the  atmosphere,  at  considerable 
depths  below  the  level  of  ground  water  the  atmospheric  pressure  may  be 
neglected;  and  the  pressure,  and  therefore  the  solubility  of  underground 
gases  in  water,  is  almost  directly  proportional  to  the  depth  below  the  level 
of  ground  water.  For  instance,  at  a  depth  of  only  100  meters  below  the 
level  of  ground  water  the  pressure  of  the  atmosphere  is  only  one-tenth 
that  of  the  water  pressure;  at  a  depth  of  1,000  meters  it  is  only  one 

"•Ostwald,  W.,  Solutions,  translated  by  M.  M.  Pattiaon  Muir;  Longmans,  Green  &  Co.,  New  York, 
1891,  p.  9. 


RELATIONS  OF  PRESSURE  AND  SOLUTION.         71 

one-hundredth,  and  at  still  gi.ater  depths  the  fraction  of  pressure  due  to 
the  atmosphere  is  insignificant. 

But  in  order  that  saturation  for  any  gas  corresponding  to  the  pressure  at 
any  given  depth  shall  occur,  it  is  necessary  that  a  sufficient  amount  of  gas 
shall  there  exist.  Gases  may  be  produced  below  the  level  of  ground  water 
by  the  chemical  reactions,  as  by  the  liberation  of  carbon  dioxide  in  the 
process  of  silication.  Later  it  will  be  seen  (see  Chapter  VIII,  pp.  677-679) 
that  this  is  one  of  the  fundamental  processes  of  the  lower  physical-chemical 
zone.  It  follows  from  the  above  that  at  depth  the  amount  of  carbon 
dioxide  or  other  gas  in  solution  per  unit  of  water  may  be  many  score  times 
greater  than  near  the  surface.  The  pressure  of  carbon  dioxide  at  the 
surface  is  only  about  0.0006  of  an  atmosphere.  The  water  pressure  at  a 
depth  of  1,000  meters  is  almost  100  atmospheres;  therefore  the  amount  of 
free  carbon  dioxide  which  may  be  held  in  solution,  if  pressure  were  the 
only  factor  concerned,  might  be  166666  times  as  great  as  that  held  in 
solution  in  the  belt  of  weathering. 

But  it  must  be  remembered  that,  as  shown  below,  the  increase  of 
temperature  due  to  increase  of  depth  somewhat  reduces  this  multiple. 

It  should  be  remembered  also  that  carbon  dioxide  combines  with 
water,  producing  carbonic  acid,  and  the  amount  of  this  compound  which 
may  be  held  in  solution  at  the  surface  of  ground  water  is  not  dependent 
upon  the  pressure  of  the  atmospheric  carbon  dioxide.  But  it  is  evident 
that  deep  ground  waters,  where  the  pressure  is  great,  may  hold  a  vastly 
greater  quantity  of  carbon  dioxide  than  can  be  held  in  solution  near  the 
level  of  ground  water. 

As  already  pointed  out,  the  law  which  obtains  in  reference  to  geological 
work  is  that  the  activity  of  the  carbon  dioxide  increases  in  direct  ratio  with 
its  quantity. 

The  theoretical  conclusion  that  the  action  of  carbon  dioxide  would  be 
increased  by  pressure,  and  consequent  greater  quantity,  has  been  experi- 
mentally verified  by  Mueller"  and  Struve,  who  found  that  strong  pressure 
increased  the  action  of  carbon  dioxide  in  the  decomposition  of  the  silicates 
more  than  did  increase  of  time.  • 

a  Mueller,  Richard,  Untersuehungen  iiber  die  Einwirkung  des  kohlensiiurehaltigen  Wassers  auf 
einige  Mineralien  und  Gesteine:  Tschermaks  mineral.  Mittheil.,  vol.  7,  1877,  p.  47. 


72  A  TREATISE  ON  METAMORPHISM. 

The  temperature. — Increase  of  temperature  generally  results  in  decrease  of 
solubility  of  a  gas."  Increase  in  temperature  with  depth,  or  because  of 
volcanism,  lessens  the  solubility  of  gases  in  ground  water,  and  to  this 
extent  works  against  the  effect  of  increased  pressure. 

solids  in  solution. — There  is  still  another  factor  which  enters  to  a  slight 
extent  into  the  solubility  of  gases.  Water  holding  solids  in  solution,  in 
most  cases,  absorbs  less  of  a  gas  at  a  given  pressure  than  does  pure  water.6 
However,  the  solutions  near  the  surface  are  ordinarily  so  dilute  that  this 
law  is  probably  not  important,  but  at  depth  it  may  be  of  some  consequence 
in  working  against  the  effect  of  increased  pressure. 


SOU;TI<»  OF  SOLIDS  IN  OROUXD  WATER. 


Where  a  solid  is  placed  in  a  liquid  some  or  all  of  it  dissolves,  and  thus 
forms  a  homogeneous  mixture  composed  of  the  two,  or  a  solution. 

It  has  been  found  that  if  a  liquid  be  placed  in  a  vessel  having  two  com- 
partments separated  by  a  membrane  through  which  the  solvent  but  not  the 
dissolved  substance  may  pass,  when  a  soluble  compound — for  instance, 
sugar — is  dissolved  in  the  liquid  in  one  of  the  compartments,  pressure 
against  the  membrane  is  produced.  This  pressure  has  been  called  osmotic 
pressure,  to  distinguish  it  from  ordinary  gas  pressure,  known  as  vapor 
pressure.  According  to  vau't  Hoff,  the  osmotic  pressure  "is  independent 
of  the  nature  of  the  solvent,  and  in  general  obeys  the  laws  of  gases."  That 
is  to  say,  "the  osmotic  pressure  is  proportional  to  the  concentration;  the 
osmotic  pressure  is  proportional  to  the  absolute  temperature;  the  same 
osmotic  pressure  can  be  obtained  by  equimolecular  quantities  of  the  most 
various  substances  in  the  same  solvent;  the  osmotic  pressure  is  exactly  the 
same  as  the  gas  pressure  which  would  be  observed  if  the  solvent  were 
removed  and  the  dissolved  substance  were  left  filling  the  same  space  in  the 
gaseous  state  at  the  same  temperature."0  These  somewhat  sweeping  state- 
ments need  various  modifications.  For  instance,  where  the  solutions  are 
very  concentrated  the  molecules  in  solution  are  believed  to  be  so  close  to 

aQstwald,  W.,  Outlines  of  general  chemistry,  translated  by  James  Walker,  Macmillan  &  Co., 
London,  2d  ed.,  1895,  p.  121. 

&Ostwald,  op.  cit.,  p.  121. 

"Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
pp.  134-137.  Otwald,  W.,  Solutions,  translated  by  M.  M.  Pattison  Muir;  Longmans,  Green  &  Co.. 
New  York,  1891,  pp.  112-117. 


CONDITIONS  OF  SOLIDS  IN  SOLUTION.  73 

one  another  that  molecular  attraction  produces  an  effect,  and  in  this  case 
the  osmotic  pressure  does  not  vary  directly  as  the  concentration.  But, 
Cameron  says,  in  so  far  as  the  molecules  in  solution  are  sufficiently  sepa- 
rated so  that  they  may  act  as  a  gas,  "the  volume,  pressure,  and  temper- 
ature relations  are  dependent  only  upon  the  number  of  molecules  involved."0 

Since  all  of  these  relations  are  the  same  as  the  laws  controlling  the 
behavior  of  gases,  it  is  held  by  many  physical  chemists  that  when  a  solid 
passes  into  solution  it  is  transformed  to  a  gas.  Under  this  explanation  the 
osmotic  pressure  is  a  gaseous  pressure.  "The  kinetic  energy  of  the 
molecules  of  the  dissolved  substance  is  equal  to  that  of  the  gas  at  the  same 
temperature ;  and,  moreover,  as  the  kinetic  energies  of  the  molecules  of  the 
dissolved  subtance  and  of  the  solvent  must  agree,  because  these  molecules 
are  in  immediate  contact,  it  follows  also  that  the  kinetic  energy  of  the 
molecules  of  the  liquid  must,  on  the  whole,  be  the  same  as  that  of  gaseous 
molecules  at  the  same  temperature.'"' 

If  the  above  theory  be  correct,  it  follows  that  the  solution  of  solids  in 
liquids  is  similar  to  that  of  gases  in  liquids;  for  in  both  cases  the  compound 
when  dissolved  is  in  the  form  of  a  gas;  and  the  geological  work  of  under- 
ground water,  whether  the  solutions  be  produced  by  a  mingling  of  gases 
and  water,  solids  and  water,  or  the  three  combined,  could  be  considered  as 
a  unit.  (See  pp.  63-64.) 

In  case  a  salt  dissolved  in  water  be  an  electrolyte,  under  the  dissocia- 
tion theory  it  is  separated  into  ions  to  some  extent.  If  this  be  so,  the 
number  of  dissolved  particles  is  represented  by  the  number  of  ions  plus  the 
number  of  undissociated  molecules.  Therefore  in  very  dilute  solutions, 
where  the  dissociation  is  held  to  be  complete,  the  number  of  dissolved 
particles  and  consequently  the  osmotic  pressure,  is  doubled  in  the  case  of 
a  salt  of  a  monad  acid  with  a  monad  base.  Thus  the  law  of  equal  gaseous 
pressure  for  equal  number  of  molecules  is  believed  by  many  to  still  hold 
good.  For  instance,  if  NaCl  dissociates  into  the  ions  Na  and  Cl,  or  KOH 
into  the  ions  K  and  OH,  thus  giving  twice  as  many  molecules  as  in  the 
case  of  a  compound  which  does  not  ionize,  under  the  law  the  osmotic 
pressure  is  twice  as  great  as  that  of  the  compound  which  does  not  dissociate. 

"Cameron,  F.  K.,  Application  of  theory  of  solutions  to  the  study  of  soils:  Report  No.  64,  Field 
Operations  of  Division  of  Soils,  1899,  U.  S.  Dept.  of  Agric.,  1900,  p.  144. 
&0stwald,  op.  cit.,  p.  148. 


74  A  TREATISE  ON  METAMORPHISM. 

The  conclusions  of  van't  Hoff,  Ostwald,  and  others  in  reference  to 
osmotic  pressure  being  due  to  gaseous  pressure  of  tht  dissolved  substances 
have  never  been  accepted  by  Mendeleeff,  and  have  recently  been  strongly 
opposed  by  Kahlenberg.  Certainly  there  are  many  discrepancies  between 
the  observations  made  as  to  the  amount  of  osmotic  pressure  and  the  amount 
which  the  pressure  should  be  under  the  gas  law.  But.  so  far  as  the 
observations  of  geology  show,  I  see  nothing  that  controverts  or  confirms 
van't  Hoff's  theory.  In  studying  the  work  of  underground  solutions  I 
have  been  unable  to  discover  any  criteria  which  will  separate  the  work  of 
gases  in  water  solutions  from  the  work  of  solids  in  water  solutions.  So 
far  as  geology  is  concerned,  solutions  of  gases  in  water  and  solutions  of 
solids  in  water  can  not  be  discriminated.  It  has  been  held  by  some  that 
the  presence  of  fluorite  and  other  minerals  is  evidence  of  gaseous  action, 
but,  as  yet,  I  have  not  been  able  to  find  valid  evidence  offered  by  any 
author  for  this  conjecture.  It  may  be  that  gases  dissolved  in  water  and 
solids  dissolved  in  water  are  held  in  solution  in  consequence  of  chemical 
affinity,  as  held  by  Mendeleeff,  or  they  may  be  in  solution  as  gases,  as 
held  by  van't  Hoff,  but  in  either  case  the  manner  of  action  of  the  two  is 
the  same,  and  therefore  there  is  no  warrant  for  attributing  the  development 
of  fluorite,  tourmaline,  etc.,  to  the  presence  of  "  miueralizers  "  in  the  sense 
that  these  compounds  are  the  products  of  the  action  of  gases  as  opposed  to 
water  solutions. 

When  a  soluble  solid  is  placed  in  a  liquid  solvent  it  at  once  begins  to 
dissolve.  The  temperature  and  pressure  remaining  constant,  if  an  excess 
of  the  solid  be  present  after  a  sufficient  time  there  is  no  further  decrease 
in  the  amount  of  the  solid  present,  nor  is  there  any  increase.  When  this 
state  is  reached  the  solution  is  saturated. 

When  a  solid  is  in  a  saturated  solution,  and  therefore  constant  in 
amount,  even  if  temperature  and  pressure  remain  constant  it  does  not  follow 
that  no  interchange  takes  place  between  the  dissolved  and  solid  salt.  The 
kinetic  theory  of  solutions  leads  to  the  conclusion  that  many  molecules  are 
released  from  the  solid  to  the  solution,  and  pass  from  the  solution  into  the 
solid,  but  these  amounts  balance.  This  is  well  illustrated  by  sugar  solu- 
tions. If  finely  pulverized  sugar  be  placed  in  the  bottom  of  a  saturated 
sugar  solution  and  sugar-covered  threads  be  suspended  in  the  solution, 
sticks  of  rock  candy  will  be  formed.  The  crystals  of  the  candy  grow  at 


GROWTH  OF  LARGE  CRYSTALS.  75 

the  expense  of  the  sugar  below,  which  is  being  constantly  taken  into  solu- 
tion and  deposited  as  crystals  about  the  string1;  and,  therefore,  although  the 
solution  is  continuously  saturated,  there  is  continuous  solution  and  deposi- 
tion. Even  if  no  sugar-coated  strings  were  placed  in  the  sugar,  after  a 
time  it  would  be  found  to  be  coarser  grained  or  to  have  recrystallized. 
Thus  the  constant  interchange  between  a  saturated  solution  and  that  of  an 
adjacent  solid  is  certain. 

The  change  occurs  under  the  law  by  which  large  crystals  grow  at  the 
expense  of  small  ones.  In  order  that  crystals  shall  grow  in  a  solvent,  it  is 
necessary  that  the  solutions  shall  be  saturated  or  supersaturated  at  the 
immediate  place  of  crystal  growth.  Since  underground  there  is  always  a 
superabundance  of  many  materials  as  compared  with  the  amount  of  water, 
we  may  suppose  that  at  a  moderate  depth  below  the  surface,  and  especially 
in  the  smaller  spaces,  where  movement  is  very  slow  (see  pp.  138-146),  the 
solutions  are  often  saturated.  It  is  well  known  that  the  growth  of  larger 
crystals  at  the  expense  of  smaller  ones,  under  conditions  of  saturation  and 
superabundance  of  material,  goes  on  more  rapidly  in  proportion  as  the 
temperature  is  high  and  the  pressure  is  great.  The  principle  is  taken 
advantage  of  in  the  chemical  laboratory  in  the  production,  before  nitration,  of 
a  coarse  precipitate  by  boiling  or  other  means.  During  the  process  the  finer 
particles  of  the  precipitate  are  dissolved  and  the  coarser  ones  are  enlarged 
at  their  cost.  The  growth  of  the  large  crystals  at  the  expense  of  the  small 
ones  is  due  to  the  fact  that  the  smaller  crystals  are  somewhat  more  soluble 
than  the  larger.  The  explanation  of  this  change,  as  given  by  Ostwald,a 
lies  in  the  "surface  tension  which  exists  on  the  boundary  surfaces  between 
solids  and  liquids,  as  on  those  between  liquids  and  gases — the  so-called 
free  surfaces  of  liquids.  This  tension  acts  so  that  the  surfaces  in  question 
are  reduced  in  size,  with  the  consequent  enlargement  of  individual  crystals 
(the  total  amount  of  precipitate  remaining  practically  unaltered),  i.  e.,  with 
the  coarsening  of  the  grains."  During  the  change,  for  a  given  volume  of 
solid  the  lessening  of  the  total  surface  of  the  crystals,  and  consequently  the 
lessening  of  the  surface  tension,  results  from  the  fact  that  the  surfaces  are 
small  in  proportion  as  the  individuals  are  large.  For  a  given  volume  of  a 
substance  the  surfaces  of  the  crystals  are  inversely  as  their  diameters.  (See 

a  Ostwald,  W.,  The  scientific  foundations  of  analytical  chemistry,  translated  by  George  McGowan, 
Macmillan  &  Co.,  London,  1895,  p.  22. 


76  A  TREATISE  ON  METAMORPHISM. 

p.  98).  The  increase  in  the  size  of  the  crystals,  lessening  the  surface 
tension,  may  be  considered  as  a  transfer  of  potential  into  kinetic  energy. 
This  passes  into  heat  and  is  dispersed  under  the  apparently  general  law  of 
the  dissipation  of  energy.  Why  the  tendency  to  the  transformation  of  all 
forms  of  energy  into  heat  and  the  dissipation  of  heat  should  be  a  law  of 
nature  it  is  not  my  purpose  here  to  discuss.  But  such  the  law  seems  to 
be,  and  in  its  application  we  carry  the  causal  sequence  as  far  as  we  are 
now  able. 

The  growth  of  large  individuals  at  the  expense  of  small  ones  in  ground 
water  is  of  the  most  profound  significance  in  the  metamorphism  of  rocks. 
It  is  illustrated  by  the  secondary  enlargement  of  minerals  and  by  the  por- 
phyritic  crystals  which  frequently  develop  in  schists  and  gneisses,  such  as 
the  porphyritic  crystals  of  feldspar,  hornblende,  garnet,  staurolite,  etc. 
(See  pp.  643-644,  699-700.) 

The  above  principle  in  reference  to  the  growth  of  large  crystals  at  the 
expense  of  small  ones  is  very  clearly  applicable  to  the  growth  of  segregations 
of  minerals  of  a  certain  kind  as  compared  with  smaller  segregations.  If, 
for  instance,  at  one  place  there  be  a  mineral  aggregate,  this,  so  far  as  the 
surface  tension  and  the  free  surface  of  liquids  are  concerned,  acts  as  a  unit 
and  tends  to  draw  to  itself  the  material  of  smaller  aggregates  or  of  individual 
mineral  particles.  For  aggregates  which  do  not  have  crystal  boundaries 
the  form  which  would  be  assumed  under  ideal  conditions  is  spherical. 
This  principle  of  the  growth  of  large  aggregates  at  the  expense  of  small 
ones  is  illustrated  by  chert  nodules.  (See  pp.  816-818.) 

The  quantity  of  a  solid  which  can  be  dissolved  in  aqueous  solutions 
depends  upon  the  compounds  present,  the  pressure,  and  the  temperature. 
When  the  limit  of  solubility  is  reached  the  solution  is  said  to  be  saturated. 

COMPOUNDS   PRESENT. 

Theoretically  all  compounds  are  soluble  to  some  extent  in  water.  This 
statement  applies  to  all  natural  compounds;  that  is,  the  minerals  of  nature 
are  elements,  oxides,  or  salts  which  are  soluble  in  water.  No  substance  is 
wholly  insoluble  in  the  ground  solutions,  even  at  the  ordinary  temperatures 
and  pressures.  Tliis  statement  is  illustrated  by  the  solution  of  quartz  and 
the  more  refractory  silicates  at  the  surface."  Under  surface  conditions 

a  Hayes,  C.  W.,  Solution  of  silica  under  atmospheric  conditions:  Bull.  Geol.  Soc.  America,  vol.  8,. 
1S97,  pp.  214-217. 


MUTUAL  INFLUENCE  OF  COMPOUNDS.  77 

quartz  grains  are  sometimes  etched  by  meteoric  waters,  and  the  decompo- 
sition and  partial  solution  of  the  refractory  silicates  is  universal.  Under 
conditions  of  deep-water  circulation  solution  of  quartz  and  the  refractory 
silicates  may  be  accomplished  with  relative  rapidity.  This  is  illustrated 
by  the  Calumet  and  Hecla  conglomerate,  many  of  the  pebbles  of  which 
have  been  partly  or  even  completely  dissolved  and  the  space  once  occupied 
by  them  taken  by  copper.0 

Since  underground  solutions  always  contain  a  number  of  compounds, 
and  often  many,  the  influence  of  one  compound  upon  the  solubility  of 
another  is  of  consequence  in  various  ways.  For  instance,  when  several 
compounds  are  present,  a  unit  quantity  of  water  will  not  dissolve  as  much 
of  a  given  salt  as  it  would  if  it  were  alone.  But  if  a  number  of  units  of 
water  are  each  saturated  with  a  single  salt,  and  the  solutions  are  mingled 
without  chemical  reaction,  the  mixture  is  capable  of  taking  additional  quan- 
tities of  the  salts  into  solution.  In  other  words,  a  unit  of  solution  simul- 
taneously saturated  with  each  of  several  compounds  contains  a  greater 
total  of  solids  than  a  unit  of  solution  saturated  with  fewer  of  these  com- 
pounds, but  less  of  any  individual  salt  than  it  would  were  it  saturated  with 
that  salt  alone.6 

In  the  ground  solutions  the  different  compounds  frequently  react  upon 
one  another,  and  therefore  important  modifications  in  the  above  statement 
are  necessary,  as  is  explained  under  "Precipitation,"  pp.  113-123. 


RELATIONS   OK    SOLUTION    AND    PRE&Sl'RE. 


In  general,  the  volume  of  the  solvent  plus  that  of  the  dissolved 
compound  is  greater  than  that  of  the  solution.  For  a  given  quantity  of 
the  solid  the  contraction  is  greater  the  more  of  the  solvent  is  used.c  In 
some  cases,  however,  the  volume  of  the  dissolved  compound  and  solvent  is 
less  than  that  of  the  solution,  or  expansion  results  from  dissolving  the  solid. 
Ammonium  chloride  in  water  is  an  illustration  of  this  case.  From  the  fore- 
going relations  we  obtain  a  rule  as  to  the  relations  of  pressure  to  solubility.* 
In  the  common  case  in  which  the  volume  of  the  solution  is  less  than  that  of 

«Pumpelly,  R.,  The  paragenesis  anil  derivation  of  copper  and  its  associates  on  Lake  Superior:  Am. 
Jour.  Sci.,  3d  ser.,  vol.  2,  1871,  p.  34. 

»0stwald,  W.,  Solutions,  translated  by  M.  M.  Pattison  Muir;  Longmans,  Green  &  Co.,  lle-.v  York, 
1891,  pp.  83,  84. 

"Ostwald,  W.,  op.  cit.,  p.  82. 

<*Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macuiillan  &  Co.,  London, 
1895,  p.  567. 


78 


A  TREATISE  ON  METAMOKPHISM. 


solvent  and  solid,  pressure  increases  solubility;  for  in  that  case  solution 
tends  to  bring  the  molecules  nearer  together  and  works  in  conjunction  with 
the  pressure.  A  mixture  of  water  and  ice  furnishes  an  excellent  illustration 
of  this  principle.  At  any  moment  the  volume  of  the  water  is  less  than 
that  of  the  equivalent  water  and  ice.  Hence  pressure  promotes  solution  and 
prevents  freezing,  or  in  other  words,  crystallization.  In  the  reverse  case, 
that  in  winch  the  volume  of  the  solution  is  greater  than  that  of  solvent  and 
solid,  pressure  decreases  the  solubility,  the  reason  being  the  reverse  of  that 
of  the  previous  case. 

The  above  law  may  be  illustrated  by  fig.  1.  A  given  amount  of  salt, 
say  10  cc.  in  volume,  may  be  supposed  to  be  placed  in  90  cc.  of  water,  and 
the  salt  be  of  such  a  nature  as  to  saturate  the  water  at  that 
temperature  and  pressure.  Before  solution  begins  the 
space  occupied  is  100  cc.  After  solution  this  space  may 
be  greater  or  less  than  100  cc.,  say  105  cc.  or  95  cc. ;  that 
is,  the  water  surface  instead  of  being  at  aa  will  be  at  cc  or 
Hb.  If  it  be  at  bl,  where  the  volume  is  less,  and  the  pres- 
sure be  increased,  an  additional  amount  of  salt  ma}  be 
added  and  taken  into  solution.  If  it  be  at  cc,  and  the 
pressure  be  increased,  a  part  of  the  salt  already  in  solution 
will  be  precipitated  from  the  solution. 

It  is  well  known  that  the  solubility  of  calcium  car- 
bonate and  of  some  other  carbonates  is  increased  by  pres- 
sure." It  is  a  fair  inference  from  Barus's  work  that  the 
solubility  of  the  silicates  is  also  increased  by  pressure. 
Barus6  found  that  when  soft  glass  is  dissolved  in  water  at  temperatures 
above  210°  C.,  the  volume  is  20  to  30  per  cent  less  than  the  two  sepa- 
rately. This  glass  is  one  which  contains  alkalies,  alkaline  earths,  and 
lead,  and  therefore  is  somewhat  similar  in  composition  to  many  natural 
silicates.  The  carbonates  and  the  silicates  are  the  dominant  compounds 
in  underground  solutions.  The  solubility  of  many  other  salts,  besides 
the  carbonates  and  silicates,  occurring  underground  is  increased  by  pres- 
sure. Therefore,  in  the  majority  of  the  complex  underground  solutions 

"Lindgren,  W.,  Gold-quartz  veins  of  Nevada  City  and  Grass  Valley,  California:  Seventeenth 
Ann.  Kept.  U.  8.  Geol.  Survey,  pt.  2,  1896,  pp.  176-178. 

6Barus,  C.,  Hot  water  and  soft  glass  in  their  therinodynarnic  relations:  Am.  Jour.  Sci.,  4th 
ser.,  vol.  9,  1900,  p.  173. 


FIG.  1.— Change  of  vol- 
ume resulting  from  so- 
lution, and  relations  of 
solution  and  preaiure. 


RELATION  OF  SOLUTION  AND  TEMPERATURE.        79 

the  totals  of  the  salts  in  solution  are  in  general  increased  by  pressure, 
and  the  volumes  of  the  solution  are  less  than  those  of  the  salts  and  solvents 
separately. 

RELATIONS    OF   SOLUTION    AND    TEMPERATURE. 

The  relations  of  solution  and  temperature  have  three  phases;  first,  the 
speed  of  solution;  second,  the  quantity  of  material  which  may  be  held  in 
solution;  third,  the  relations  of  solution  to  absorption  and  liberation  of  heat. 

speed  of  solution. — The  speed  of  solution  is  commonly  increased  greatly  by 
rise  of  temperature."1  A  slight  increase  in  temperature  may  increase  the  rate 
of  solution  out  of  all  proportion  to  the  absolute  change  in  temperature.  At 
temperatures  above  100°  C.,  and  especially  above  185°  C.,  the  activity  of 
water  may  increase  to  an  amazing  degree.  The  rapid  solution  of  glass,  by 
Barus,6  at  temperatures  about  185°  C.  illustrates  this.  At  any  temperature 
solution  will  continue  until  the  point  of  saturation  is  reached,  but  this  state 
will  be  attained  at  high  temperatures  in  but  a  small  fraction  of  the  time 
required  at  low  temperatures.  For  instance,  to  saturate  an  underground 
solution  with  the  refractory  silicates  or  sulphides  at  ordinary  temperatures 
might  require  months,  or  even  years,  while  to  saturate  them  at  temperatures 
above  185°  C.  might  require  only  an  equal  number  of  minutes,  or  at  most, 
hours.  The  capacity  of  water  for  action  at  high  temperatures  combined 
with  pressure,  considered  above,  is  adequate  to  explain  the  complete 
recrystallizatiori  of  great  volumes  of  rock.  (See  pp.  749-751.) 

Quantity  of  material  which  may  be  held  in  solution. The    effect    of     temperature     UpOll 

the  quantity  of  material  which  may  be  held  in-solution  does  not  admit  of  a 
simple  general  statement."  For  most  substances  moderate  increase  of 
temperature  gives  greater  capacity  for  solution;  but  for  many  substances 
there  exists  a  temperature  at  which  there  is  the  maximum  capacity  for 
solution,  and  the  amount  of  material  which  may  be  held  in  solution  at 
higher  and  lower  temperatures  is  less  than  this  maximum.  The  quanti- 
tative relations  of  solution  and  temperature  at  ordinary  pressure  between 

oNernst,  W.,  Theoretical  chemistry,  translated  by  C.  8.  Palmer,  Macmillan  &  Co.,  London,  1895, 
p.  568. 

*  Barus,  C.,  Hot  water  and  soft  glass  in  their  thermodynamic  relations:  Am.  Jour.  Sci.,  4th  ser., 
vol.  6,  1898,  p.  270,  and  vol.  9,  1900,  pp.  167-168. 

''Ostwald,  W.,  Solutions,  translated  by  M.  M.  Pattison  Muir;  Longmans,  Green  &  Co.,  New- 
York,  1891,  pp.  55-77. 


80 


A  TREATISE  ON  METAMORPHISM. 


0°  C.  and  100°  C.  are  shown  by  fig.  2,  taken  from  Ostwald."  For  various 
substances  the  maximum  capacity  for  solution  lies  between  60°  and  140° 
C.,  and  for  many  substances  it  is  probably  below  200°  C.  It  therefore 
follows,  in  respect  to  underground  solutions,  that  a  general  statement  can 
not  be  made  as  to  how  change  of  temperature  may  affect  solubility. 
However,  it  is  highly  probable  that  up  to  temperatures  of  100°  C.,  and 
therefore  under  normal  conditions  to  depths  of  3,300  meters,  increase  of 
temperature  increases  the  average  capacity  of  underground  water  to  hold 
material  in  solution;  and  it  is  probable  that  the  average  capacity  of 
ground  water  increases  to  temperatures  considerably  above  100°  C.,  and 
therefore  to  depths  greater  than  3,300  meters.  But  when  water  passes 
downward  to  the  deeper  parts  of  the  zone  of  fracture  the  increase  in  temper- 
ature may  lessen  the  average  capacity  for  holding  material  in  solution, 

provided  the  joint  effect  of  pressure  be 
barred.  But  it  has  been  seen  that  increas- 
ing pressure  with  increasing  depth  pro- 
motes solubility.  It  is  almost  certain  that 
high  temperature  and  pressure  combined 
greatly  increase  the  capacity  of  water  for 
10°  20°  30°  40°  so0  60°  7o°  80°  90°  100°  solution.  This  is  proved  by  the  experi- 


Temp«rature. 


,j      j,  _ 

Fie.  2.-Quantitative  relations  between  solution  and      m6ntS     OI     O&VUS     UpOll     the     Solubility     Of 

glass.     He  has  shown  that  at  temperatures 

above  185°  C.  and  below  200°  C  it  is  possible  "to  impregnate  glass 
with  water  to  such  an  extent  as  to  make  it  fusible  below  200°  C.  The 
solution  occurs  with  contraction  of  bulk  relatively  to  the  ingredients  and 
increasing  compressibility."  .  .  .  "If  these  solutions  are  sufficiently 
concentrated  they  coagulate  at  ordinary  temperature  and  the  congealed 
aqueous  glass  is  not  different  in  general  appearance  from  common  glass. 
The  melting  point  of  the  coagulated  aqueous  silicate  frequently  lies  below 
200°  C.,  probably  above  1 50°  C.,  depending  on  the  glass."  And  he  con- 
cludes that  "Glass  as  a  colloid  is  miscible  in  all  proportions  with  water." b 

Since  glass  is  one  of  the  important  silicate  rocks  which  occur  in  nature, 
these  statements  are  directly  applicable  to  one  set  of  rocks.     They  may 

"Ostwald,  W.,  Grundlinien  der  anorganischen  Cheinie,  Engelmann,  Leipzig,  1900,  p.  222.. 
'>Barun,  C.,  Remarks  on  colloidal  glass:  Am.  Jour.  Sci.,  4th  ser.,  vol.  6,  1898,  p.  270.     See  also  Am. 
Jour.  Sci.,  4th  ser.,  vol.  9,  1900,  pp.  161-175. 


GRADATION  BETWEEN  LIQUIDS  AND  SOLIDS.  81 

not  be  applicable  to  the  same  extent  to  crystallized  silicate  rocks,  but  it 
seems  to  me  highly  probable  that  they  apply  in  large  measure  to  many. 
In  so  far  as  Barus's  final  conclusion  is  applicable,  there  may  result  all  grada- 
tions, from  solutions  in  which  the  water  is  the  dominant  constituent  to 
those  in  which  it  is  the  subordinate  constituent.  This  principle  of  the 
increased  quantity  of  material  which  may  be  held  in  solution  as  a  result  of 
combined  high  pressure  and  temperature  is  believed  to  possess  very  great 
significance  in  alterations  in  the  zone  of  anamorphism,  and  to  be  of  impor- 
tance in  alterations  in  the  belt  of  cementation.  (See  pp.  602-603,  659-661.) 

Relations  of  solution  to  absorption  and  liberation  of  heat. As     already    explained,    wlieil 

material  passes  into  solution  the  molecules  are  separated  and  acquire 
kinetic  energy,  and  are  believed  by  many  to  change  from  the  solid  to  the 
gaseous  form.  This  process  absorbs  heat.  On  the  other  hand,  where  the 
volume  of  the  solution  is  less  than  the  volume  of  the  solvent  and  salt  sepa- 
rately, the  molecules  of  the  solvent  and  salt  combined  are  brought  closer 
together  and  heat  is  therefore  liberated.  In  the  reverse  case,  where  the 
volume  of  the  solution  is  greater  than  that  of  the  solvent  and  salt  separately, 
the  molecules  are  pushed  farther  apart,  and  heat  is  absorbed.  If  the  com- 
pounds in  solution  separate  into  ions  this  process  is  believed  to  be  usually 
attended  by  liberation  of  heat."  Whether  there  is  a  rise  or  fall  of  tempera- 
ture of  the  solution  will  depend  upon  the  relative  values  of  these  factors. 
In  the  common  case  where  there  is  decrease  in  the  volume  as  a  result  of 
solution,  the  heat  thus  liberated  by  change  in  volume  plus  the  supposed 
heat  of  ionization  are  together  preponderant,  and  there  is,  therefore,  libera- 
tion of  heat  and  a  rise  in  temperature.  However,  in  the  case  where  there 
is  increase  in  the  volume  as  a  result  of  solution,  the  heat  thus  absorbed  and 
the  heat  absorbed  in  changing  the  salt  from  the  state  of  a  solid  to  that  of  a 
gas  is  greater  than  that  supposed  to  be  liberated  by  dissociation.  The  first 
two  factors  are  dominant,  and  there  is  usually  a  marked  absorption  of  heat 
and,  consequently,  a  fall  in  the  temperature  of  the  solution.  This  is  illus- 
trated by  the  solution  of  ammonium  chloride  in  water.  The  volume  is 
considerably  decreased  and  the  fall  in  temperature  is  very  decided. 

"Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co..  London,  1895, 
p.  562. 

MON   XLVII — 04 6 


82  A  TREATISE  OX  METAMORPHISM. 


1HFFI'SIO>. 


It  has  been  seen  that  the  molecules  of  gases  and  of  solids  when  dis- 
solved in  water  are  distributed  through  the  solution.  When  the  material 
dissolved  is  not  evenly  distributed  the  molecules  are  more  abundant  here 
and  less  abundant  there.  If  the  theory  be  true  that  the  dissolved  solids  are 
gaseous  the  molecules  would  exert  a  greater  pressure  where  more  closely 
packed.  Under  these  conditions  molecules  where  more  closely  packed 
move  toward  places  where  they  are  less  closely  packed.  This  move- 
ment is  regarded  by  many  as  the  explanation  of  osmotic  pressure. 
Kahlenberg,  however,  does  not  accept  this  explanation,  but  regards  osmotic 
pressure  as  due  to  the  "mutual  attraction  between  solvent  and  dissolved 
substance.""  Without  reference  to  either  theory  the  more  important 
conclusions  in  reference  to  diffusion  may  be  summarized. 

The  force  which  drives  the  dissolved  substances  from  place  to  place, 
and  the  velocity  with  which  a  dissolved  substance  wanders  in  a  solvent,  is 
proportional  to  the  degree  of  concentration.6  Therefore,  "the  quantity  of 
a  salt  which  diffuses  through  a  given  area  is  proportional  to  the  difference 
between  the  concentrations  of  two  areas  infinitely  near  one  another.""  In 
other  words,  diffusion  is  proportional  to  the  difference  in  strength.  The 
quantity  diffused  is  proportional  to  the  square  root  of  the  time  of  diffusion, 
and  the  distance  over  which  a  determinate  concentration  extends  is  also 
proportional  to  the  square  root  of  the  time  of  diffusion.*1  Several  salts  in  a 
solution  diffuse  almost  independently  of  one  another,  each  at  its  own  specific 
rate.'  At  20°,  according  to  Ostwald,  there  is  twice  as  much  diffusion  as  at 
0°,  and  at  40°  twice  as  much  as  at  20°/  When  a  solution  is  in  equilibrium 
the  concentration  of  the  solution  varies  inversely  as  the  temperature.  It 
follows  that  when  the  temperature  of  the  solution  varies,  equilibrium  is 
obtained  not  by  equal  distribution  of  the  solutes,  but  by  unequal  distri- 
bution. If  the  temperature  be  the  same  throughout  a  solution  with  equal 

«  Kahlenberg,  Louis,  The  theory  of  electrolytic  dissociation  as  viewed  in  the  light  of  facts  recently 
ascertained:  Bull.  Univ.  of  Wisconsin  No.  47,  1901,  p.  349. 

&Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
pp.  143-144. 

cOstwald,  W.,  Solutions,  translated  by  M.  M.  Pattison  Muir;  Longmans,  Green  &  Co.,  New  York. 
1891,  p.  120. 

<*  Solutions,  cit.,  p.  135. 

« Solutions,  cit,  p.  139. 

/Solutions,  cit.,  p.  13C. 


SLOWNESS  OF  DIFFUSION.  83 

distribution  of  the  dissolved  compounds,  a  deviation  from  uniformity  in  the 
temperature  of  the  solution  will  disturb  the  equilibrium  and  result  in 
unequal  distribution  of  the  dissolved  substances.0 

The  values  of  the  coefficient  of  diffusion  (D)  of  certain  substances  in 
water  solutions  at  various  temperatures  are  given  by  the  following  table 
from  Nernst:6  (The  table  gives  the  number  of  grams  of  the  dissolved 
substance  which  will  pass  in  one  day  through  a  section  of  1  sq.  cm.  when 
the  difference  in  concentration  of  the  cross  section  1  cm.  apart  amounts  to 
1  gram  in  a  cubic  centimeter.) 

Hates  of  diffusion  of  certain  substances  in  water  solutions  at  various  temperatures. 

Temp.  D. 

Hydrochloric  acid 0. 0  1.4 

Do 11.0  1.84 

Nitric  acid 9. 0  1.  75 

Sulphuric  acid 7. 5  1. 04 

Acetic  acid 14.0  .81 

Potassium  hydroxide 13. 5  1. 66 

Sodium  hydroxide 8. 0  1. 96 

Ammonium  hydroxide 4.  5  1.06 

Sodium  chloride 6. 0  .75 

Ammonium  chloride 17.5  1.31 

Potassium  chloride 9. 0  .66 

Barium  chloride 8. 0  .65 

Potassium  nitrate 7. 0  .92 

Sodium  nitrate 13.0  .90 

Silver  nitrate 7. 5  .90 

Lead  nitrate 12. 0  .70 

Urea 7. 5  .81 

Chloral  hydrate ;     9. 0  .55 

Mannite 10. 0  .38 

This  table  shows  that  diffusion  is  extremely  slow.  The  slowness 
with  which  diffusion  occurs  is  due,  according  to  Nernst,  to  "the  resistant 
friction  experienced  by  the  dissolved  substance  in  its  movement  through 
the  solvent." c  This  friction  is  very  great,  because  the  molecules  themselves 
are  exceedingly  small. 

Later  it  will  be  seen  that  the  process  of  diffusion  is  of  very  considerable 
importance  in  the  migration  of  compounds  in  ground  water.  (See  pp. 
636-639.)  This  is  illustrated  by  the  very  important  process  of  solution 
and  deposition  or  recrystallization. 

«Ostwald,  Solutions,  cit.,  pp.  150-151. 

&  Nernst,  W.,  Theoretical  chemistry,  translated  by  0.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
p.  144. 

c Nernst,  op.  cit.,  p.  145. 


84  A  TREATISE  ON  METAMOKPHISM. 

PRINCIPLES  OF   CHEMICAL    REACTIONS   APPLICABLE   TO   GROUND    WATERS. 

GENERAL  STATEMENT. 
DEFINITIONS. 

Before  taking  up  chemical  reactions  it  is  advisable  to  give  a  number 
of  elementary  definitions. 

"Compounds  whose  aqueous  solutions  contain  the  hydrogen  ion  (H) 
are  termed  acids,  and  those  which  contain  the  hydroxyl  ion  (OH)  bases."" 
To  illustrate,  HC1  is  an  acid;  NaOH  is  a  base.  When  the  hydrogen  ion 
united  with  one  or  more  uonmetallic  elements  is  mingled  in  solution  with 
the  hydroxyl  ion  united  with  a  metal  a  double  reaction  occurs,  resulting 
in  the  union  of  the  hydrogen  ions  with  the  hydroxyl  ions,  forming  water, 
and  the  union  of  the  nonmetallic  parts  of  the  compound  with  the  metallic 
parts.  This  latter  union  forms  a  salt.  For  example — 

HCl+NaOH=NaCl+H2O. 
H2COs+2NaOH=Na2C03+2H20. 

The  acids  and  salts  which  contain  only  a  single  nonmetallic  element  are 
called  binary  compounds.  The  acids  and  salts  which  contain  two  non- 
metallic  elements  are  called  ternary  compounds.  For  example,  HC1  is  a 
binary  acid;  NaCl  is  a  binary  salt;  H2CO3  is  a  ternary  acid;  Na2CO3  is 
a  ternary  salt.  Compounds  having  the  composition  of  acids,  bases,  and 
salts  may  be  separated  from  solution  as  solids,  and  of  course  all  of  these 
solids  may  pass  into  solution. 

Some  salts  also  contain  a  certain  amount  of  acid,  and  such  salts  are 
called  acid  salts.  For  instance — 

NajCO,+H2CO5=2NaHCO8. 

The  latter  compound  is  acid  sodium  carbonate.  On  the  other  hand,  some  salts 
contain  some  additional  base,  and  such  salts  are  called  basic.  For  example, 
Fe2(S04)3  may  be  united  with  Fe2(OH)6,  producing  mFe2(SO4)3.nFe2(OH)6. 
This  compound  is  basic  ferric  sulphate. 


DIHSOCUTION. 


In  explaining  chemical  reactions  the  theory  of  dissociation  as  advocated 
by  Arrhenius,  Ostwald,  Nernst,  and  others  is  followed  for  the  most  part 

"Ostwald,  W.,  The  scientific  foundations  of  analytical  chemistry,  translated  by  George  McGowan, 
Macmillan  &  Co.,  London,  1895,  p.  117. 


THEORY  OF  DISSOCIATION.  85 

This  theory  is  firmly  placed  in  the  text-books.  No  opinion  is  expressed 
by  me  as  to  its  correctness.  Indeed,  I  have  no  right  to  any  opinion  on  the 
subject.  As  already  pointed  out,  this  theory  has  been  vigorously  opposed 
by  Kahlenberg,  but  as  yet  that  author  has  offered  no  constructive  theory 
to  take  its  place.  I  therefore  follow  the  theory  of  the  standard  text-books 
so  far  as  necessary  to  show  how  it  would  apply  to  the  work  of  ground  solu- 
tions if  it  prove  to  be  true  ;  but  so  far  as  practicable  I  make  the  statements 
in  such  form  that  they  will  be  correct  even  if  the  theory  of  free  ions  and 
reactions  between  such  ions  is  finally  abandoned. 

Under  the  theory  of  dissociation  the  superiority  of  water  as  a  solvent 
for  chemical  interchanges  is  regarded  as  largely  due  to  the  fact  that  the 
dissolved  substances  are  separated  into  their  ions  to  a  greater  degree  than 
in  any  other  solvent.  To  the  fact  of  active  reactions  in  water,  whatever 
their  cause,  are  very  largely  due  the  profound  changes  which  occur  in  rocks 
through  the  medium  of  water  solutions.  ' 

Under  the  theory  of  dissociation  water  solutions,  acids,  bases,  and  salts 
separate  into  their  ions.  For  instance,  HC1  separates  into  the  free  ions  H 
and  Cl;  NaOH  separates  into  the  free  ions  Na  and  OH;  and  NaCl  into  the 
free  ions  Na  and  Cl.  However,  in  solutions  the  dibasic  acids  are  supposed 
to  separate  into  free  ions  somewhat  differently  from  what  might  be  expected. 
For  instance,  it  might  be  expected  that  H2C03  would  separate  into  the  free 
ions  H2  and  CO3,  but  it  is  supposed  to  separate  thus  : 


Other  dibasic  acids  are  thought  to  dissociate  in  a  similar  manner.  However,. 
if  the  dibasic  acid  be  very  strong  the  compound  ion  may  again  break  up. 
Thus,  H2SO4  is  thought  to  first  break  up  into  the  free  ions  H  and  HSO4, 
and  the  latter  to  break  up  into  the  free  ions  H  and  SO4,  so  that  these  would 
be  the  ions  present  in  the  water.  But  in  the  case  of  the  weak  acid,  carbonic, 
it  is  thought  that  the  last  change  does  not  take  place,  and  that  the  free  ions 
remain  H  and  HCO3. 

Ostwald  regards  the  absence  of  the  second  stage  of  dissociation  as  the 
explanation  of  the  peculiar  characteristics  of  carbonic  acid."  Since  carbonic 
acid  is,  next  to  silica,  the  most  important  rock-making  acid,  the  manner  in 
which  it  breaks  up  is  of  great  consequence  in  metamorphism. 

"Ostwald,  W.,  Grundlinien  der  anorganischen  Chemie,  Engelmann,  Leipzig,  1900,  pp.  276-278,. 
397-398. 


86  A  TREATISE  ON  METAMORPHISM. 

HYDROLYSIS. « 

Under  the  theory  of  dissociation,  not  only  do  acids,  bases,  and  salts 
separate  into  ions  in  water  solutions,  but  the  water  itself  is  believed  to 
dissociate  to  a  very  small  extent,  according  to  the  equation  H20=rH-f-OH, 
thus  simultaneously  forming  free  hydrogen  and  hydroxyl.  If  this  be  true 
the  hydrogen  ions  and  the  hydroxyl  ions  coexist  and  water  solutions  to  a 
small  extent  contain  free  acids  and  free  bases  at  the  same  time.  The 
excellence  of  water  as  an  agent  for  reactions  between  the  substances  it 
holds  in  solution  is  held  to  be  partly  due  to  hydrolysis. 

When  strong  bases  and  acids  are  in  solution  the  amount  of  their  dis- 
sociation is  believed  to  be  so  much  greater  than  that  of  water  that  the 
dissociation  of  the  latter  is  of  little  consequence.  But  if  a  very  strong  base 
be  united  with  a  weak  acid  the  solution  will  give  an  alkaline  reaction,  and 
this  is  regarded  as  showing  the  presence  of  free  hydroxyl  ions  or  of 
Hydrolysis.  For  instance,  if  the  strong  base,  sodium,  be  united  with  the 
weak  acid,  carbonic,  and  a  water  solution  be  made,  it  is  held  that  hydrolysis 
will  take  place  to  some  extent,  thus: 

Na5COs+H2O=NaHCOs+NaOH. 

It  is  supposed  that  NaHCO3  breaks  up  into  the  ions  Na  and  HC03,  and 
the  NaOH  into  the  ions  Na  and  OH.  Therefore,  in  a  solution  of  Na2CO3 
in  water  the  coexistent  ions  are  thought  to  be  H,  HC03,  Na,  and  OH. 
Since  the  base,  NaOH,  is  stronger  than  the  acid,  HC03,  the  separation  into 
the  ions  is  thought  to  be  the  explanation  of  the  alkaline  reaction. 

Cameron  has  shown  that  sodium  silicate  in  solution  gives  an  alkaline 
reaction,  and  his  explanation  is  that  this  compound  is  hydrolized  in  a  man- 
ner precisely  similar  to  that  of  sodium  carbonate.6  Not  only  do  solutions 
of  sodium  silicate  give  alkaline  reactions,  but  Clarke  has  shown"  that  many 
natural  mineral  silicates,  when  treated  with  pure  water,  show  an  alkaline 
reaction.  The  following  gave  permanent  alkaline  reactions:  Phlogopite, 
oligoclase,  albite,  cancrinite,  sodalite,  analcite,  natrolite,  pectolite,  apophyl- 
lite,  segirite.  The  following  gave  more  or  less  distinct  colorations  to  the 
phenolphthaleiu  indicator,  but  in  time  faded:  Muscovite,  lepidolite,  ortho- 

"Ostwald,  W.,  Grundlinien  der  anorganischen  Chemie,  Engelmann,  Leipzig,  1900,  pp.  254-257. 

^Cameron,  F.  K.,  Application  of  the  theory  of  solutions  to  the  study  of  soils:  Rept.  No.  64,  Field 
Operations  of  the  Division  of  Soils,  1899,  U.  8.  Dept.  of  Agric.,  1900,  p.  169. 

«  Clarke,  F.  W.,  Alkaline  reaction  of  some  natural  silicates:  Jour.  Am.  Chem.  Soc.,  vol.  20,  1898, 
pp.  739-742. 


HYDROLYSIS  OF  COMPOUNDS.  87 

clase,  leucite,  nephelite,  spodumene,  scapolite,  laumontite,  stilbite,  chabazite, 
heulandite,  thomsonite."  If  the  theory  of  dissociation  be  true,  this  shows  that 
the  silicates  are  hydrolized,  thus : 

R2SiO4+4HOH =2R(OH  )2+H4Si04. 

However,  according  to  Kahlenberg  and  Lincoln,  the  H4SiO4  does  not  dis- 
sociate into  the  radicals  H  and  SiO4,  but  forms  colloidal  silicic  acid.6  Thus 
this  compound  is  inert  and  the  reaction  reverses  only  to  a  small  extent  and 
under  favorable  conditions,  and  the  hydrate  of  the  alkali  metal  gives  an 
alkaline  reaction. 

In  a  similar  manner  hydrolysis  is  held  to  occur  in  water  solutions  of  the 
strong  base  sodium  with  the  weak  acid  hydrosulphuric,  thus: 

Na2S+H2O=NaHS+NaOH. 

Cameron  further  states  that  hydrolysis  is  to  be  expected  in  the  case  of  the 
aluminates  and  ferrates/  When  a  strong  acid  is  united  with  a  weak  base 
the  solution  gives  an  acid  reaction,  and  this  is  also  explained  by  dissociation, 
the  free  acid  supposed  to  result  from  hydrolysis  being  stronger  than  the 
weak  base.d 

Since  the  three  most  abundant  acids  of  nature  are  silicic,  carbonic,  and 
hydrosulphuric,  all  weak,  hydrolysis,  if  true,  is  a  reaction  of  fundamental 
importance  in  metamorphism. 

REACTIONS. 

When,  after  a  number  of  chemical  substances  are  brought  together, 
and  especially  when  they  are  united  by  a  solvent,  interactions  between 
them  may  occur  which  after  a  time  appear  to  cease.  When  the  conditions 
have  become  such  that  there  is  no  increase  or  decrease  in  the  amount  of 
any  one  of  the  chemical  compounds,  the  system  is  in  a  condition  of 
chemical  equilibrium.8  When  two  substances  in  solution,  A  and  B,  react 
upon  each  other  so  as  to  produce  two  other  substances,  C  and  D,  if 
solutions  of  C  and  D  are  mixed  they  in  turn  will  react  upon  each  other  to 

«  Clarke,  cit.,  pp.  740-741. 

6  Kahlenberg,  L.,  and  Lincoln,  A.  T.,  Solutions  of  silicates  of  the  alkalies:  Jour.  Phys.  Chem.,  vol. 
2,  1898,  pp.  77-90. 

e Cameron,  cit.,  p.  169. 

<*Ostwald,  Grundlinien,  cit.,  pp.  276-278,  397. 

"Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
pp.  35§-356. 


88  A  TREATISE  ON  METAMORPHISM. 

produce  more  or  less  of  the  substances  A  and  B.°  That  is,  the  reaction  is 
reversible  to  a  greater  or  less  degree.  To  illustrate,  if  two  solutions,  one 
of  them  containing  MgSO4  and  the  other  Na2CO3,  come  together,  the  ions 
are  Mg,  Na,  SO4,  and  CO3.  A  part  of  the  Mg  will  unite  with  the  CO3, 
producing  MgCO3,  and  a  part  of  the  Na  will  unite  with  the  SO4,  producing 
Xa2SO4.  Vice  versa,  if  solutions  of  Na2S04  and  MgC03  are  mingled  in  a 
similar  manner,  MgSO4  and  Na-jCOs  will  be  produced.  The  reversible 
reaction  may  be  briefly  expressed  thus: 


MgSO,- 

The  sign  71  means  that  the  equations  may  be  read  from  left  to  right 
or  from  right  to  left. 

These  are  the  facts:  The  ions  do  interchange  between  compounds 
whenever  a  chemical  reaction  takes  place.  Just  how  and  why  they  inter- 
change is  another  matter,  upon  which  there  is  not  agreement.  Under  the 
theory  of  dissociation  the  interchange  takes  place  through  the  medium  of 
the  free  ions.  The  free  ions  of  a  compound  A  are  held  to  collide  with  the 
free  ions  of  the  compound  B,  and  thus  produce  the  compound  C  and  D, 
and  vice  versa.  Or,  in  the  specific  case  above  given,  of  MgSO4  and  Na^COs, 
the  free  Mg  ions  collide  with  the  free  CO3  ions  and  produce  MgCO3,  and 
the  SO4  ions  collide  with  the  Na  ions  and  produce  Na^SO^  From  the 
MgCO3  and  Na2SO4,  MgSO4  and  Na2CO3are  reproduced  in  a  similar  manner. 
Whether  the  theory  of  chemical  reactions  through  free  ions  be  of  any  value  or 
not,  it  seems  probable  that  free  ions  exist  for  a  moment  when  the  interchange 
takes  place.  To  illustrate,  it  seems  hardly  probable  that  the  Mg  is  united 
to  the  SO4  and  the  C03  at  the  same  time.  If  the  Mg  lets  go  of  the  S04  to 
attach  itself  to  the  C03,  for  that  instant  the  ion  Mg  is  free.  The  same  is 
true  of  each  of  the  other  ions,  Na,  SO4,  and  C03,  at  the  instant  of  inter- 
change. Hence  the  question  at  issue  is  the  cause  of  the  interchange.  Does 
it  take  place  as  the  result  of  contact  of  free  ions  produced  by  dissociation, 
or  does  the  chemical  affinity  of  the  Mg  for  the  CO3  cause  a  portion  of  it  to 
leave  the  stronger  acid  radical  SO4  for  the  weaker  acid  radical  CO3,  etc.? 
But  this  is  a  question  for  chemists  to  settle.  The  problem  is  stated  here 
because  it  is  one  of  such  fundamental  importance  in  metamorphism. 

But  whatever  the  cause,  reversible  reactions  are  a  certainty,  and  it  will 

oNernst,    W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
pp.  356-357. 


REVERSIBLE  REACTIONS.  89 

be  seen  that  certain  reactions  of  the  zones  of  katamorphism  are  reversed  in 
the  zone  of  anamorphism,  and  vice  versa.  But  it  should  be  remembered 
that  to  close  the  cycle  in  any  case,  or,  in  other  words,  to  make  a  trans- 
formation from  left  to  right  and  then  from  right  to  left,  thus  completely- 
reversing  any  reaction,  requires  the  expenditure  and  dissipation  of  energy. 

The  most  general  law  controlling  chemical  systems  is  expressed  by  Le 
Chatelier  as  follows:  "Every  change  of  one  of  the  factors  of  an  equilibrium 
occasions  a  rearrangement  of  the  system  in  such  a  direction  that  the  factor 
in  question  experiences  a  change  in  a  sense  which  is  contrasted  with  the 
original  change."  ° 

Nernst  remarks  that  this  law  reminds  one  of  the  principle  of  action  and 
reaction.  Put  in  another  way,  it  may  be  said  that  any  chemical  change,  by 
the  mere  fact  of  its  occurrence,  sooner  or  later  renders  the  conditions  less 
favorable  for  its  continuance.  To  illustrate,  if  the  increase  in  volume 
demanded  by  the  reaction  becomes  too  great,  this  may  stay  the  reaction. 
For  example,  if  calcium  acetate  and  copper  acetate  be  placed  together  in  a 
very  strong  vessel  and  but  little  additional  space  be  left,  the  reaction  resulting 
in  the  expansion  of  volume  will  go  on  until  the  pressure  becomes  so  great  as 
to  stay  the  reaction.6  Also,  if  in  a  closed  vessel  a  large  amount  of  calcium 
carbonate  be  heated  it  will  give  off  carbon  dioxide.  But  as  the  amount  of 
CO2  increases,  and  the  pressure  therefore  accumulates,  the  reaction  will  be 
retarded  and  finally  cease.  At  this  stage  the  CO2  formed  unites  with  the 
CaO,  producing  CaC03,  as  fast  as  CaCO3  decomposes  and  produces  CaO 
and  CO2. 

If  the  heat  as  the  result  of  a  reaction  becomes  too  great,  this  will  stay 
the  reaction.  For  instance,  at  low  temperatures  CO  will  completely  unite 
with  0,  producing  CO2;  but  if  the  temperature  becomes  too  high,  as  a 
result  of  the  change  the  reaction  will  be  stayed  or  cease  altogether.  A 
case  of  much  greater  geological  consequence  is  that  of  hydration  and 
dehydration.  A  comparatively  low  temperature  is  favorable  to  hydration 
of  minerals.  However,  a  very  moderate  temperature — anything  above 
110°  C. — at  ordinary  conditions  of  pressure  is  likely  to  stay  the  reaction 
of  hydration,  or  even  to  reverse  this  process  and  produce  dehydration. 

« Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
p.  567. 

6 Jones,  H.  C.,  On  the  increasing  importance  of  inorganic  chemistry:  Science,  new  ser.,  vol.  8, 
1898,  p.  930. 


90  A  TREATISE  ON  METAMORPHISM. 

Van't  Hoff  states  the  general  law  controlling  chemical  systems  in 
another  way:  "A  transformation  will  take  place  of  itself  only  in  case  it  is 
in  a  position  to  do  a  positive  amount  of  work.  If  the  amount  of  work 
done  is  negative,  the  transformation  can  take  place  of  itself  only  in  the 
opposite  sense.  If  the  work  done  is  zero,  it  can  take  place  in  neither 
sense." " 

EQUILIBRIUM. 

When  the  ions  of  any  compound,  A,  unite  with  the  ions  of  another 
compound,  B,  so  as  to  produce  C  and  D,  just  as  fast  as  the  ions  of  C  unite 
with  the  ions  of  D  to  produce  A  and  B  the  conditions  are  those  of  equi- 
librium, or  of  chemical  statics.  When  either  change  takes  place  faster 
than  the  other,  these  are  the  conditions  of  chemical  reactions,  or  of  chemical 
kinetics.6  When  the  two  solutions,  A  and  B,  were  first  mingled,  the  con- 
ditions would  be  those  of  chemical  kinetics  for  a  time — that  is,  until  a 
certain  amount  of  C  and  D  had  been  produced.  However,  when  the 
amount  of  C  and  I)  is  sufficiently  great,  so  that  they  react  upon  each  other 
to  produce  A  and  B  as  fast  as  A  and  B  react  to  produce  C  and  1),  the 
conditions  are  those  of  chemical  statics,  or  equilibrium.  But  it  is  plain 
that  this  does  not  mean  that  chemical  activity  has  ceased  or  that  there  is 
real  quiescence.  Interchange  is  taking  place  all  the  time ;  but  as  this  inter- 
change is  compensatory,  no  heat  effect  is  produced,  and  the  total  quantity 
of  each  of  the  compounds  present  remains  the  same.  The  equilibrium  is 
therefore  really  dynamic. 

HOMOGENEOUS  AND  HETEROGENEOUS  SYSTEMS. 

A  chemical  system  is  homogeneous  "  when  it  has  the  same  physical  and 
chemical  nature  at  every  point."  When  this  is  not  the  case  it  is  heteroge- 
neous." A  solution  is  therefore  homogeneous.  If  a  solid  substance  be  also 
present,  the  system  is  heterogeneous.  A  heterogeneous  system  consists  "  in 
the  intimate  association  of  different  complexes,  each  of  which  is  homoge- 
neous in  itself,  such  as  solid  salts  and  saturated  solutions.'"2  Each  of  these 
complexes  is  called  a  phase  of  the  system.  "The  condition  of  equilibrium 
of  a  heterogeneous  system  is  independent  of  the  relative  quantity  by  weight 

"Jones,  H.  C.,  On  the  increasing  importance  of  inorganic  chemistry;  Science,  new  series,  vol.  8, 
1898,  p.  930. 

&Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
pp.  358-360. 

«Nernst,  cit.,  p.  357. 
t,  cit,  p.  391. 


HETEROGENEOUS  SYSTEMS.  91 

in  which  each  phase  is  present  in  the  system.""  To  illustrate,  if  an  excess 
of  salt  be  in  a  solution,  so  that  it  is  saturated,  and  an  additional  amount 
of  salt  be  added,  this  does  not  in  the  least  change  the  quantity  of  salt 
held  in  a  given  volume  of  the  solution.  Therefore  the  equilibrium  in  a 
saturated  solution  is  independent  of  the  amount  of  undissolved  salt  in  the 
solution.  It  follows  that  in  a  heterogeneous  system  "  the  condition  of 
equilibrium  is  independent  of  the  relative  mass  of  each  of  the  phases."6  A 
simple  case  of  heterogeneous  equilibrium  is  that  between  ice  and  liquid 
water,  or  between  liquid  water  and  water  vapor.  "  For  a  definite  external 
pressure  there  corresponds  a  definite  temperature  at  which  the  two  systems 
can  exist  beside  each  other ;  thus  ice  and  water  are  coexistent  at  atmos- 
pheric pressure  at  0°  C.;  and  liquid  water  and  water  vapor,  at  atmospheric 
pressure  and  at  100°  C.  If  we  change  the  external  pressure,  at  a  tempera- 
ture which  is  kept  constant,  or  if  we  change  the  temperature,  at  an  external 
pressure  which  is  kept  constant,  then  the  reaction  advances'"1  to  equilibrium 
in  one  direction  or  the  other.  "  The  process  is  ended  as  soon  as  the 
expansive  force  of  the  evaporating  or  dissolving  substance  is  held  in  equi- 
librium by  the  gas  pressure  of  the  vaporized  molecules  or  by  the  osmotic 
pressure  of  the  dissolved  molecules,  respectively."0 

NATURE  AND  SPEED  OF  REACTIONS. 

The  fundamental  principle  of  chemical  dynamics  is  that  chemical  action 
is  proportional  to  the  active  mass.d  This  is  the  law  of  mass  action. 

The  speed  of  a  chemical  reaction  which  occurs  under  any  given  con- 
ditions depends  upon  the  compounds,  the  strength  of  the  solutions,  the 
mechanical  action,  and  the  heat.  Hence  each  of  these  features  requires  con- 
sideration. 

THE  COMPOUNDS. 

The  reactions  depend  upon  the  compounds  present,  or,  in  other  words, 
upon  the  nature  of  the  ions  composing  them ;  for  the  conditions  under  which 
two  ions,  A  and  B,  unite  may  be  different  from  those  under  which  one  of 
these  ions  will  unite  with  a  third,  as  A  with  C,  or  different  from  those  under 
which  two  other  ions,  C  and  D,  unite.  In  order  that  ions  shall  unite  in 
solution  they  must  meet  or  come  within  the  limits  of  molecular  attraction  of 

"Nernst,  cit.,pp.  391-392. 
fcNernst,  cit.,  p.  393. 
"Nernst,  cit.,  p.  403. 

rfOstwald,  W.,  Outlines  of  general  chemistry,  translated  by  Jamea  Walker,  Macmillan&  Co.,  2d  ed., 
London,  1895,  p.  292. 


92  A  TREATISE  ON  METAMORPHISM. 

one  another  under  certain  definite  conditions  which  are  peculiar  to  each  sub- 
stance. Therefore  not  every  time  such  a  meeting  occurs  are  compounds 
formed.  The  ratio  between  meeting  and  union  in  the  case  of  any  two  com- 
pounds is  a  constant,  which  can  be  compared  with  the  constant  of  any 
other  two  compounds,  each  pair  of  which  lias  its  constant.  This  is  merely 
another  statement  of  the  old  law  that  different  substances  have  different 
affinities  for  one  another,  and  it  is  well  known  that  the  chemical  affinities 
are  developed  only  when  the  molecules  are  in  immediate  contact  with  one 
another. 

The  ions  which  are  present  in  ground  waters  in  any  given  case 
largely  depend  upon  the  character  of  the  adjacent  rocks.  In  a  lime- 
stone region,  for  instance,  the  water  may  quickly  take  into  solution  all 
the  calcium  and  magnesium  it  can  hold,  considering  the  acids  present. 
Under  such  circumstances  the  acid  ions  will  be  mainly  balanced  by  the  cal- 
cium and  magnesium.  The  other  substances,  such  as  sodium  and  potas- 
sium, perhaps  in  more  readily  soluble  forms  than  the  calcium  and  magnesium, 
will  be  largely  kept  from  going  into  solution,  or  if  in  solution  will  be  partly 
thrown  down,  because  these  substances  are  obliged  to  compete  for  the  acid 
radicals  with  the  vastly  greater  number  of  calcium  and  magnesium  molecules. 
Is  it  not  possible  that  the  agricultural  advantage  of  having  calcium  and  mag- 
nesium abundantly  in  the  soil  is  largely,  or  at  least  partly,  due  to  the  fact 
that  the  presence  of  these  soluble  substances  in  abundance  prevents  the 
solution  and  washing  out  of  the  elements  potassium  and  sodium  which  the 
plants  need! 

Ostwald  divides  the  bases  into  strong,  moderately  strong,  and  weak." 
The  alkalies  and  alkaline  earths,  with  the  exception  of  magnesium,  are 
strong  bases;  magnesium  is  a  moderately  strong  base;  iron  and  aluminum 
are  weak  bases — of  the  two  aluminum  is  the  weaker.  It  follows  that, 
other  things  being  equal,  in  underground  solutions  the  alkalies  and  alkaline 
earths,  with  the  exception  of  magnesium,  largely  take  possession  of  the  acids. 
To  a  less  extent  this  is  true  of  magnesium,  and  to  a  still  smaller  degree  of 
iron  and  aluminum.  Thus  we  have  the  partial  explanation  of  the  relative 
solubilities  of  the  bases  in  the  belt  of  weathering.  In  this  belt  the  alkalies 
are  dissolved  to  the  greatest  extent;  next  in  order  comes  calcium,  then 
magnesium,  and  finally  iron  and  aluminum.  (See  p.  518.) 

a  Ostwald,  W.,  The  scientific  foundations  of  analytical  chemistry,  translated  by  George  McGowan, 
Macmillan  &  Co.,  London,  1895,  pp.  55-56. 


WEAKNESS  OF  ACIDS  COMPENSATED  BY  QUANTITY.  93 

Ostwald  divides  the  acids  into  strong,  moderately  strong,  weak,  and 
very  weak.  The  acids  H2SO4,  HC1,  and  HNO3,  are  strong  acids.  The 
acids  H2S03  and  H3PO4  are  moderately  strong.  The  acids  H2S,  H3B03, 
and  H2CO3  are  weak  acids.  The  acids  of  silica  are  very  weak." 

The  strong  acids  H2SO4,  HC1,  HNO3,  when  present  in  ground  solu- 
tions, as  they  sometimes  are,  of  course  take  possession  of  the  bases  in 
proportion  to  their  quantity.  However,  in  the  crust  of  the  earth  strong 
acids  are  not  abundant  on  the  average,  although  under  exceptional  con- 
ditions, as  in  volcanic  districts,  they  may  be  rather  plentiful.  Also  the 
moderately  strong  acids  H2SO3  and  H3PO4  are  not  abundant,  although 
phosphoric  acid  is  rather  widespread.  Of  the  weak  acids  H2S  and  H3B03 
are  not  plentiful.  The  two  great  acids  of  nature  are  carbonic  and  silicic 
acids,  and  the  major  contest  in  the  rocks,  so  far  as  the  acids  are  concerned, 
is  between  the  weak  carbonic  acid  and  the  very  weak  silicic  acid.  These 
two  acids  are  everywhere  very  abundant  in  the  rocks.  While,  therefore, 
the  moderately  strong  and  the  strong  acids  play  a  relatively  important 
part  in  proportion  to  their  quantity,  one  weak  and  one  very  weak  acid, 
because  of  their  dominant  quantity,  under  the  law  of  mass  action  play 
the  greatest  part  in  rock  alterations;  and  in  the  contest  the  very  weak 
acid,  silicic,  holds  its  own  against  the  weak  acid,  carbonic,  partly  because 
its  far  greater  abundance  compensates  for  its  relative  weakness.  The  fact 
of  the  formation  of  carbonates  and  the  simultaneous  decomposition  of 
the  silicates  under  surface  conditions  the  world  over  is  well  known. 
(See  pp.  163,  473-486.)  The  partial  explanation  of  the  phenomena  is  the 
relative  abundance  of  carbonic  acid  under  the  conditions  in  the  zone  of 
katamorphism.  As  shown  in  another  place  (see  p.  479),  the  reaction  is  also 
one  which  liberates  heat,  and  this  is  a  favorable  factor  in  the  process. 

In  the  zone  of  anamorphism,  where  the  pressure  is  great,  the  reaction 
of  the  upper  zone  is  reversed.  (See  pp.  173-178,  677-679.)  The  replace- 
ment of  carbonic  by  silicic  acid  results  in  decrease  in  volume  (see  p.  177). 
Therefore,  under  the  great  pressures  of  the  zone  of  anamorphism, 
the  relative  volumes  of  the  original  and  secondary  compounds  is  a  most 
important,  probably  dominant,  factor  in  the  process.  But  also  it  is 
probable  that  at  the  high  temperatures  and  pressures  which  obtain  in  the 
lower  zone  silicic  acid  gains  strength  as  compared  with  carbonic  acid. 

«  Foundations,  cit. ,  p.  55. 


94  A  TREATISE  ON  METAMORPHISM. 

It  may  under  these  conditions  be  a  stronger  acid  than  at  the  surface, 
and  if  this  were  the  case  the  reactions  would  be  partly  explained. 
Bearing  in  this  direction  is  the  experiment  of  Bischof,  who  has  shown 
that  at  100°  C.  silicic  acid,  when  present  in  abundance,  may  partially 
replace  carbonic  acid  of  carbonates." 

Ostwald's  explanation  of  the  varying  strength  of  the  bases  and  acids  is 
based  on  the  varying  amount  of  supposed  dissociation. 

The  velocity  of  a  reaction  is  proportional  to  the  masses  of  the  active 
components,  and  according  to  Ostwald  these  are  the  free  ions.  Therefore 
the  speed  depends  upon  the  number  of  free  ions  which  are  acting.  But 
the  number  of  free  ions  which  are  present  is  dependent  upon  the  degree 
of  dissociation,  and  in  this  matter  different  compounds  vary  greatly. 
Therefore  the  degree  of  electrolytic  dissociation  of  the  various  bases  and 
acids  determines  their  respective  strengths  and  is  "  the  measure  of  the 
reaction  capacities  of  all  substances."  b 

From  this  it  follows  that  an  acid  or  base  which  is  strongly  dissociated 
is  stronger  than,  or,  in  other  words,  is  able  to  largely  replace,  an  acid  or 
base  which  is  but  slightly  dissociated;  for  the  number  of  free  ions  of  the 
stronger  compound  far  exceeds  that  of  the  weaker.  It  therefore  becomes 
important,  from  Ostwald's  point  of  view,  to  know  the  comparative  strength, 
or  the  relative  amounts  of  dissociation,  of  the  abundant  bases  and  acids 
which  occur  in  the  rocks.  According  to  Ostwald  the  strong  bases  and 
strong  acids  may  be  largely  dissociated;  the  moderately  strong  bases  and 
acids  under  ordinary  conditions  are  dissociated  to  a  much  less  extent;  the 
weak  acids,  carbonic,  hydrosulphuric,  and  boric,  are  usually  not  dissociated  to 
the  extent  of  1  per  cent;  silicic  acid  under  ordinary  conditions  is  scarcely 
dissociated  at  all. 

STRENGTH  OF  THK  SOLUTIONS. 

Saturated  and  strong  solutions  are  more  active  than  weaker  solutions; 
for  the  amount  of  the  active  compound  increases  with  the  concentration, 
but  not  in  a  simple  ratio.  Weak  solutions  are  relatively  more  active  than 
strong  solutions,  and  by  those  who  believe  in  dissociation  this  is  attributed 
to  their  nearer  approach  to  complete  dissociation;  but  the  greater  relative 
activity  of  weak  solutions  never  compensates  fully  for  the  greater  dilution. 

°  Bischof,  Gustav,  Elements  of  chemical  and  physical  geology,  translated  by  Paul  and  Drummond, 
Harrison  &  Sons,  London,  1854,  vol.  1,  p.  6. 

^Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
p.  440. 


EFFECT  OF  QUANTITY  OF  ELEMENTS.  95 

Although,  as  just  seen,  strong  bases  and  acids  have  a  great  advantage 
over  weak  bases  and  acids,  the  quantity  of  an  element  present  is  a  very 
important  factor  in  the  final  result  of  the  action  of  the  solutions  on  the  solids. 
If  a  certain  element  is  abundant  in  the  ground  solutions,  it  may  to  a  large 
extent  replace  another  element  in  the  solids,  an  element  of  the  solid  going 
into  solution  at  the  same  time.  This  may  take  place  to  a  large  extent  even 
if  the  element  in  solution  is  weaker  than  the  one  it  replaces  in  the  solid. 
For  instance,  the  relatively  weak  base,  magnesium,  when  abundant  in 
solutions,  is  known  to  replace  the  stronger  base,  calcium,  on  a  large  scale 
in  calcium  carbonate,  thus  changing  limestone  to  dolomite.  In  this  reaction, 
while  the  abundance  of  magnesium  is  a  very  important  factor,  a  number  of 
others  enter;  and  therefore  its  detailed  consideration  is  given  under  the 
process  of  rock  dolomitization.  (See  pp.  802-808.) 

MECHANICAL  ACTION. 

It  has  already  been  seen  that  no  changes  in  rocks  take  place  without 
movements  of  material,  small  or  great,  for  long  or  short  distances.  Even 
in  the  case  of  a  mineral  passing  from  one  form  to  an  allotropic  form,  there 
is  movement  of  the  molecules.  In  short,  wherever  there  is  rearrangement 
of  the  elements  there  must  be  movements. 

Mechanical  action  alone  is  one  of  the  processes  of  metamorphism  of 
the  utmost  importance.  (See  pp.  46-50.)  However,  the  effect  of  mechan- 
ical action  in  the  promotion  of  chemical  action  is  even  more  important 
than  mechanical  action  alone. 

Mechanical  action  influences  chemical  action  in  two  general  ways — the 
speed  is  promoted,  and  the  nature  of  the  reaction  is  modified. 


SPEED    OF   CHEMICAL    ACTION. 


The  speed  of  chemical  action  is  promoted  directly  by  the  deformation, 
and  indirectly  by  the  heat  liberated. 


DIRECT  DEFORMATION  EFFECT. 


As  already  shown  (pp.  49-50),  mechanical  action  produces  deformation 
in  three  different  ways — by  producing  strain  without  rupture,  strain  with 
rupture,  and  readjustment  of  the  particles 

strain  without  rupture. — When  material  is  strained  without  rupture,  even  if 
the  amount  of  deformation  be  slight,  a  great  change  in  the  molecular  con- 
stitution may  be  involved.  This  is  well  shown  by  a  common  experiment 


96  A  TREATISE  ON  METAMORPHISM. 

on  glass.  If  a  piece  of  glass,  free  from  stress,  be  placed  under  the  micro- 
scope with  crossed  nicols,  the  light  is  cut  off  because  the  glass  is  isotropic. 
If,  however,  the  glass.be  slightly  flexed,  well  within  the  elastic  limit,  it 
immediately  becomes  anisotropic,  and  brilliant  colors  flash  out.  So  far  as 
light  is  concerned — and  this  is  one  of  the  best  agents  for  giving  an  insight 
into  the  molecular  constitution  of  bodies — the  strained  glass  behaves  wholly 
different  from  unstrained  glass.  Evidently  when  glass  is  alternately  strained 
and  freed  from  strain  it  undergoes  a  profound  change  in  molecular  consti- 
tution. The  greatness  of  the  molecular  change  in  material  when  strained 
within  the  elastic  limit  is  dwelt  upon  to  show  that  such  changes  might 
greatly  affect  chemical  action;  and  it  will  be  seen  below  that  the  facts 
correspond  to  this  expectation. 

Barus  has  shown"  in  the  case  of  metals  strained  to  the  point  of  rupture 
that  a  considerable  per  cent  of  the  energy  expended  in  straining  them  is 
potentialized;  in  "glass-hard"  steel  50  per  cent,  in  brass  40  percent,  in 
copper  25  per  cent.  A  larger  percentage  of  the  energy  was  potentialized 
in  the  earlier  stages  of  strain  than  in  the  later  stages.  By  stating  that 
energy  is  poteutialized  is  meant  that  the  mechanical  equivalent  in  heat  of 
the  work  done  on  the  metals  was  only  partially  developed ;  the  remainder 
of  the  energy  is  stored  up  in  the  strained  metals.  Now,  considering  a  brittle 
substance  which  is  analogous  in  physical  characters  to  rocks,  Prince  Rupert 
drops,  the  explosion  of  a  drop  when  a  point  is  broken  shows  tiiat  a  large 
amount  of  energy  is  potentialized,  or  that  the  glass  is  in  a  high  state  of 
strain.  The  experiments  of  Barus  and  the  condition  of  the  Rupert  drop 
show  that  in  strained  materials  energy  is  probably  potentialized.  If  this 
be  true,  must  it  not  be  the  case  that  the  atoms  and  molecules  of  a  strained 
body  are  in  a  more  than  ordinarily  favorable  condition  for  chemical  action? 

Bodies  in  which  energy  is  potentialized  are  believed  to  be  in  an 
exceptionally  favorable  condition  for  chemical  action.  For  instance,  if  a 
strained  metal,  in  which  on  that  account  more  than  the  usual  amount  of 
energy  is  stored,  be  dissolved  in  an  acid,  less  than  the  usual  amount  of 
chemical  energy  is  expended,  for  the  resultant  salts  in  the  solution  have  the 
same  energy  of  combination  in  each  case.  But  in  the  strained  metal  work 
has  been  done,  the  equivalent  of  which  has  not  escaped  as  heat  during 
strain,  and  is  therefore  stored  energy.  Therefore  this  energy  is  available 

o  Barus,  C.,  The  mechanism  of  solid  viscosity:  Bull.  U.  S.  Geol.  Survey  No.  94,  1892,  pp.  107-108. 


STRAINED  MINERALS  EASILY  DISSOLVED.  97 

to  assist  the  chemical  reaction.  That  it  is  utilized  is  shown  by  the  fact 
that  the  heat  of  combination  of  the  resultant  chemical  compound  must 
be  the  same  whatever  the  condition  of  the  metal.  Hence  less  chemical 
energy  is  required  for  the  solution  of  a  strained  metal,  and  the  reaction  is 
promoted  by  the  state  of  the  strain. 

The  validity  of  this  reasoning  is  dependent  upon  the  principle  of  the 
conservation  of  energy.  As  a  result  of  my  studies  in  the  phenomena  of 
recrystallizatioii,"  I  became  convinced  that  strained  minerals  are  more 
readily  acted  upon  by  underground  solutions  than  unstrained  minerals. 
(See  pp.  690—692.)  Barus's  experiments  already  cited  suggested  the  above 
explanation.  I  then  predicted  that  experiments  would  show  that  strained 
metals  are  more  readily  acted  upon  chemically  than  unstrained  ones,  and 
asked  that  this  prediction  be  tested  experimentally.  This  Mr.  Hambuecheu 
has  done  in  reference  to  iron,  with  the  following  results: 

The  application  of  stress  to  metals  causes  an  increase  in  chemical  activity,  this 
increase  being  especially  marked  after  the  elastic  limit  has  been  reached. 

It  is  possible  to  get  a  curve  showing  the  relation  of  electro-motive  force  to 
strain  which  is  similar  to  that  of  stress  to  strain. 

There  is  a  definite  relation  between  the  electrical  potential  of  iron  toward  an 
electrolyte  and  the  amount  of  energy  stored  up  in  the  metal  through  the  application 
of  stress.6 

Thus  complete  experimental  confirmation  of  this  prediction  is  made  so 
far  as  iron  is  concerned;  and  it  can  hardly  be  doubted  that  this  illustrates 
the  general  principle  above  given. 

Applying  the  above  principles  to  strain  and  chemical  action,  it  may  be 
said  that  in  so  far  as  minerals  are  strained  either  within  or  beyond  the 

«/ 

elastic  limit,  this  potentializes  energy  and  puts  such  minerals  into  a 
condition  more  favorable  for  chemical  reactions  than  unstrained  minerals. 
All  rocks,  except  at  the  very  surface  of  the  earth,  are  under  stress,  and 
therefore  strained  to  some  extent  at  all  times.  It  is  true  that  the  amount 
of  stress  may  not  be  great  within  a  few  meters  of  the  surface;  but  with 
increase  of  depth  the  average  amount  of  stress  becomes  more  important. 
In  most  cases  of  ordinary  horizontal  rocks  near  the  surface  it  is  customary 

"Compare  Van  Hise,  C.  R.,  Metamorphism  of  rocks  and  rock  flowage:  Bull.  Geol.  Soc.  America, 
vol.  9,  1898,  p.  300. 

*  Hambuechen,  Carl,  An  experimental  study  of  the  corrosion  of  iron  under  different  conditions: 
Bull.  Univ.  of  Wisconsin  No.  42  (Engr.  ser.,  vol.  2,  No.  8),  1900,  p.  255. 

MON    XLVII — (M 7 


98  A  TREATISE  ON  METAMORPHISM. 

to  regard  them  as  practically  free  from  stress  and  strain.  However,  not 
infrequently  rapid  deformation  by  uplift  of  an  arch  or  by  fracture  when  a 
few  meters  of  load  is  removed,  as  at  the  Chicago  drainage  canal  and  at 
the  combined  lock  of  Appleton,"  shows  that  such  rocks  are  under  very  con- 
siderable stress,  and  therefore  must  be  strained. 

Not  only  are  rocks  generally  under  stress,  but  because  of  the  com- 
plexity and  variability  of  rock  compositions,  structures,  and  textures, 
wherever  rocks  are  under  stress  the  amount  of  stress  and  therefore  of  strain 
continually  varies  with  changing  direction  and  changing  position.  Variable 
amount  of  strain  is  therefore  a  universal  law.  In  so  far  as  any  mineral 
particle  is  strained  to  a  greater  degree  than  an  adjacent  mineral  particle  of 
the  same  kind  similarly  strained,  the  paiticle  under  greater  strain  is  more 
rapidly  altered  by  chemical  action.  In  so  far  as  any  portion  of  a  mineral 
particle  is  strained  to  a  greater  degree  than  another  portion  of  the  same 
particle  similarly  strained,  the  part  under  greater  strain  is  t  more  rapidly 
altered  by  chemical  action.  Finally,  for  the  same  mineral  particle  or  some 
part  of  the  same  the  strain  varies  continually  during  deformation. 

From  the  foregoing  it  follows  that  the  almost  universal  state  of  strain, 
and  the  not  less  universal  variability  in  the  amount  of  strain,  are  of  the 
most  profound  significance  in  metamorphism.  (See  Chapters  VI,  VII,  VIII.) 

strain  with  rupture — Where  deformation  produces  rupture,  another  feature 
enters,  also  favorable  to  chemical  action.  Rupture  is  favorable  to  chemical 
action  since  thereby  the  surface  exposed  to  the  underground  waters  is 
inversely  as  the  average  diameter  of  the  mineral  particles.  Granulation 
very  greatly  increases  the  surface  of  action. 

Readjustment  of  panicles — The  readjustment  of  the  rock  particles  with  refer- 
ence to  one  another  can  hardly  fail  to  give  better  opportunities  for  the 
chemical  action  of  the  ground  waters;  for  during  the  adjustment  the 
water  will  necessarily  be  moving  and  will  come  in  contact  with  a  succession 
of  mineral  particles,  and  thus  promote  chemical  interchange.  Hence  I 
conclude  that  mechanical  action  is  favorable  to  metamorphism  by  chemical 
action,  whether  the  deformation  be  strain  without  rupture,  with  rupture,  or 
merely  readjustment  of  the  rock  particles,  or,  finally,  any  combination  of 
these. 

« Cramer,  Frank,  On  the  rock  fracture  at  the  Combined  Locks  mill,  Appleton,  Wis. :  Am.  Jour. 
Sri.,  3d  sen,  vol.  41,  1891,  pp.  432^34. 


HEAT  PRODUCED  BY  MECHANICAL  ACTION.  99 


INDIRECT   HEAT   EKFEIT. 


It  is  a  well-known  law  that  mechanical  action  develops  an  equivalent 
amount  of  heat,  except  for  the  part  of  the  energy  which  is  potentialized 
It  has  already  been  seen  that  heat  is  ordinarily  favorable  to  chemical 
action.  Therefore  mechanical  action  promotes  chemical  action,  because  it 
develops  heat  and  raises  the  temperature.  Indeed,  the  heat  developed  by 
mechanical  action  is  frequently  one  of  the  most  important  favorable  con- 
ditions for  metamorphism.  It  will  be  shown  (Chapter  VIII,  p.  740)  that 
where  mechanical  action  is  strong  the  complete  recrystallizatioii  of  rocks 
may  occur  much  nearer  the  surface  than  under  quiescent  conditions.  This 
result  is  largely  attributed  to  the  rise  in  temperature  due  to  deformation, 
which  results  in  vastly  greater  efficiency  of  the  water  as  an  agent  of 
chemical  action. 

It  therefore  becomes  of  the  utmost  importance  to  consider  to  what 
extent  the  temperature  is  raised  in  the  rocks  by  mechanical  action. 

The  heat,  as  already  intimated,  is  produced  by  the  transformation  of 
work  into  heat  as  a  result  of  straining  the  rock  particles  within  the  elastic 
limit,  by  rupturing  them,  and  by  their  frictional  movements  over  one 
another.  Mallet"  has  held  that  the  heat  thus  developed  may  be  sufficient 
to  liquefy  rocks  by  aqueo-igneous  fusion.  He  thus  accounts  for  the  crys- 
tallized cores  of  many  mountain  ranges.  He  even  holds  that  the  material 
fused  by  mechanical  action  may  intrude  the  adjacent  solid  rocks.  LeConte 
follows  Mallet  in  this  belief.  It  may  be  theoretically  possible  that  rock 
material  can  be  ground  so  fine  as  to  develop  sufficient  heat  to  fuse  it. 
However,  as  explained  (Chapter  VIII,  pp.  728-732),  we  have  no  evidence  in 
the  field  that  this  has  occurred.  It  is  shown  (Chapter  VIII,  pp.  690-696),. 
that  when  the  temperature  of  water-saturated  rocks  rises  a  certain  amount, 
readjustment  occurs,  not  by  mechanical  subdivision  and  grinding  of  the 
particles  over  one  another,  but  by  recrystallization.  The  process  is  thus 
chemical,  not  mechanical,  and  the  expenditure  of  energy  and  the  conse- 
quent development  of  heat  are  far  less  than  by  the  former  process.  How- 
ever, it  is  probable  that,  as  a  result  of  the  interior  kneading  of  rocks,  the 
temperature  may  be  materially  increased,  perhaps  several  hundred  degrees 
beyond  the  normal  temperature  which  obtains  as  a  result  of  the  depth  of 

"Mallet,  Robert,  Volcanic  energy;  an  attempt  to  develop  its  true  and  cosmical  relations:  Philos. 
Trans.  Royal  Soc.  London,  vol.  163,  1873,  pp.  147-227. 


100  A  TREATISE  ON  METAMORPHISM. 

burial.     And  it  is   certain  that  the  temperature   can  be  very  materially 
increased,  and  therefore  that  the  chemical  activity  is  enormously  increased. 


NATURE    OF   THE   CHEMICAL    REACTIONS. 


Pressure  influences  chemical  reactions  under  the  following  law:  If  a 
chemical  system  be  compressed  at  a  constant  temperature,  there  follows  a 
displacement  of  the  equilibrium  in  that  direction,  which  is  associated  with 
a  diminution  of  volume.  This  law  in  relation  to  pressure  and  chemical 
activity  may  be  stated  in  a  more  general  form,  as  follows:  "Those  chemical 
forces  are  strengthened  by  compression  which  condition  a  diminution  of 
volume;  and  those  chemical  forces  are  weakened  by  compression  which 
condition  an  increase  in  volume.''0  In  other  words,  so  far  as  pressure 
influences  chemical  reactions,  changes  go  on  in  directions  which  produce 
smaller  volumes.  Therefore  pressure  at  all  times  and  places  is  influencing 
chemical  reactions  in  the  direction  of  the  production  of  more  condensed 
systems.  It  has  been  seen  (Chapter  II,  pp.  48—49)  that  pressure  alone, 
without  the  presence  of  solutions,  may  produce  reactions  under  this  law. 
However,  in  nature,  the  vast  majority  of  reactions  under  the  law  are 
accomplished  through  the  agency  of  water.  The  importance  of  water  in 
this  connection  is  well  illustrated  by  Spring's  experiments  upon  the  con- 
solidation of  clay  when  dry  and  wet.  By  pressure  upon  moist  clay 
confined  in  a  cylinder  he  was  able  to  consolidate  the  clay  into  a  body  as 
compact  as  a  piece  of  shale — indeed,  so  compact  that  it  was  difficult  to 
scratch  it  with  the  finger  nail.  But  using  the  same  pressure  upon  dry  clay 
he  produced  a  substance  so  little  consolidated  that  it  was  easily  scratched 
with  the  finger  nail.  In  the  case  of  the  moist  clay,  he  attributed  the  consoli- 
dation to  the  escape  of  the  plastic  material  about  the  piston,  and  to  the 
precipitation  of  material  from  solution  at  the  moment  of  escape.6  Spring's 
explanation  therefore  does  not  introduce  chemical  readjustment  of  the  com- 
pounds. However,  it  will  be  seen  that  pressure  does  promote  chemical 
interchange,  producing  compounds  which  are,  on  the  average,  denser  than 
the  original  ones.  This,  as  will  be  shown  on  the  following  pages,  is 
believed  to  be  a  dominant  process  for  a  great  many  chemical  reactions 

"Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  Co.,  New  York,  1895, 
p.  567. 

&Tolman,  C.  F.,  jr.,  Professor  Spring  on  the  physics  and  chemistry  of  solids:  Jour.  Geol.,  vol.  6, 
1898,  p.  323. 


CHANGE  OF  VOLUME  BY  RECRYSTALLIZATION.  101 

resulting  from  pressure  as  the  chief  motive  force;  and  it  may.be  that 
chemical  interchange  is  one  of  the  processes  which  explain  the  consolidation 
of  the  clay  in  Spring's  experiment, 

In  the  rocks  a  smaller  volume  may  result  in  either  of  two  ways: 
Material  may  be  taken  into  solution  and  deposited  in  a  more  compact 
form  without  change  in  chemical  composition,  or  with  change  in  chemical 
composition. 

SMALLER   ROCK   VOLUME  AS  THE   RESULT  OF  SOLUTION  AND  DEPOSITION   WITHOUT  CHANGE  IN  CHEMICAL  COMPOSITION. 

It  has  already  been  explained  (pp.  77—78)  that  pressure  promotes  solu- 
tion in  case  the  volume  of  the  solution  is  less  than  that  of  the  solvent  and 
solid,  and  that  pressure  promotes  precipitation  in  case  the  volume  of  the 
solution  is  greater  than  that  of  the  solvent  and  solid.  Thus  the  solubility 
of  a  salt  increases  with  pressure,  provided  the  dissolving  is  associated  with 
a  contraction  of  the  volume  of  the  solution  plus  the  salt;  and,  conversely, 
the  solubility  decreases  if  the  separation  of  the  salt  (from  the  solution)  is 
associated  with  a  diminution  of  the  volume  of  the  system."  In  ground 
solutions  the  general  law  is  that  the  volume  of  the  solution  is  less  than  that 
of  the  substances  dissolved  and  the  water.  It  follows  from  this  law  that 
pressure  in  rocks,  the  interstices  of  which  are  filled  with  water,  promotes 
recrystallization  and  condensation. 

The  production  of  a  smaller  rock  volume  without  change  in  chemical 
composition  may  occur  where  the  recrystallization  and  condensation  take 
place  without  change  of  minerals,  and  where  the  recrystallization  and 
consolidation  take  place  with  change  of  minerals. 

Recrystallization  and  condensation  without  change  of  minerals. As    aU     illustration    of    the 

principle,  we  may  consider  a  stratum  of  unconsolidated  crystallized  calcium 
carbonate  over  which  is  a  layer  of  water  saturated  with  calcium  carbonate. 
Inasmuch  as  the  calcium  carbonate  is  porous,  the  water  in  the  rock  is  free 
to  move  and  is  under  the  pressure  of  the  hydrostatic  column  above  it. 
The  particles  of  CaC03  are  under  this  pressure,  and  also  that  of  the  solid 
above  All  the  water  in  the  crevices  and  pores  small  enough  to  hold  water 
by  capillarity  is  under  both  the  pressure  of  the  water  and  in  part  that  of 
the  rock.  This  water  is  saturated  under  this  pressure,  and  it  can  hold  more 
substances  in  solution  than  the  water  under  less  pressure.  An  interchange 

"Nerast,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  Co.,  New  York,  1895, 
p.  567. 


102  A  TREATISE  ON  METAMORPHISM. 

is  constantly  carried  on  between  the  free  and  the  capillary  water,  and  as  the 
capillary  water  becomes  free  it  is  supersaturated  and  deposits  some  of  its 
load  in  the  interstices  of  the  rock.  But  gravity  ever  pulls  the  material 
downward,  and  although  this  process  is  not  rapid,  it  is  continuous,  and  in 
course  of  time  the  particles  are  cemented.  A  solidified  and  recrystallized 
limestone  is  produced.  Evidently  the  greater  the  pressure  the  more  rapid 
and  complete  is  this  change. 

Another  example  of  solidification  without  change  in  mineral  composi- 
tion is  the  change  of  snow  or  separate  ice  crystals  where  mingled  with 
water  to  solid  ice,  as  at  the  head  of  glaciers.  Ice  has  its  melting  point  low- 
ered by  pressure.  Where  the  granules  are  under  more  than  the  average 
pressure  some  of  them  melt.  The  water  flows  out  into  the  free  spaces  and 
is  again  frozen.  Or,  as  expressed  above,  under  more  pressure  more  of  the 
ice  is  dissolved  in  the  water  than  under  less  pressure.  When  the  pressure 
is  relieved  in  the  more  open  spaces  the  ice  is  reprecipitated."  As  the 
process  goes  on  the  particles  are  finally  cemented.  This  process,  like  that 
of  the  recrystallization  of  limestone,  is  continuous,  and  finally  the  separated 
snow  granules  are  transformed  to  continuous  ice. 

Recrystallization  and  condensation  with  change  of  minerals ReCl'yStallizatioil     and     CO11- 

densation  with  change  of  minerals  but  without  change  in  chemical  composi- 
tion may  take  place  by  precisely  the  same  processes  as  already  given.  The 
resultant  minerals,  where  the  inducing  cause  is  pressure,  are  more  compact 
than  the  original  minerals.  Illustrating  this  principle,  pressure  induces 
the  transformation  of  amorphous  calcium  carbonate  to  calcite.  Similarly, 
pressure  may  induce  the  transformation  of  many  other  amorphous  substances 
to  crystalline  forms.  Pressure  also  induces  minerals  to  change  to  forms 
having  higher  specific  gravities.  Thus  pressure  tends  to  transform  tridymite, 
sp.  gr.  2.28-2.33,  to  quartz,  sp.  gr.  2.653-2.660;  and  marcasite,  sp.  gr. 
4.85-4.90,  to  pyrite,  sp.  gr.  4.95-5.10.  (See  pp.  220-221,  215.) 

We  may  also  safely  argue  that,  where  the  pressure  is  great,  minerals 
are  not  likely  to  crystallize  in  forms  having  low  specific  gravities.  Thus 
under  great  pressure  it  is  to  be  expected  that  silica  will  crystallize  as 
quartz  and  not  as  tridymite.  Doubtless  this  principle  explains  why 
quartz  is  always  found  in  the  plutonic  rocks,  and  why  tridymite  often  is 

a  Le  Chatelier,  in  Theoretical  chemistry,  by  W.  Nernst,  p.  654.     Zeitechr.  phys.  Chemie,  vol.  9, 
1892,  p.  335. 


VOLUME  DECREASED  BY  RECKYSTALLIZAT1ON.      103 

found  in  the  volcanic  rocks.  The  plutonic  rocks  crystallize  under  condi- 
tions of  great  pressure,  while  the  volcanic  rocks  crystallize  under  conditions 
of  moderate  or  slight  pressure.  It  would  be  interesting  to  know  the- 
relations  of  quartz  and  tridymite  in  the  matter  of  depth  in  the  lavas,  and 
therefore  in  reference  to  pressure  at  the  time  of  crystallization. 

SMALLER  VOLUME  AS  THE   RESULT  OF  SOLUTION  AND  REDEPO8ITION  WITH  CHANGE  IN  CHEMICAL  COMPOSITION. 

Pressure  inducing  chemical  reactions  involving  changes  in  chemical 
composition  may  produce  crystallization  and  condensation  of  amorphous 
compounds  and  recrystallization  and  condensation  of  crystallized  com- 
pounds. 

Crystallization  and  condensation  of  amorphous  compounds. 111       general       the       amOrpllOUS 

compounds  occupy  more  volume  than  their  complex  crystalline  equivalents. 
"Therefore,  since  the  crystallized  state  is  generally  that  which  takes  the 
smallest  volume,  pressure  aids  crystallization." a  According  to  Delesse,  in 
passing  from  the  crystalline  to  the  glassy  state,  granite  decreases  in  density 
9  to  11  per  cent,  syenite  8  to  9  per  cent,  diorite  6  to  8  per  cent,  dolerite  5 
to  7  per  cent,  and  trachyte  3  to  5  per  cent.6  Thus  glass  occupies  from  3  to 
1 1  per  cent  more  volume  than  the  equivalent  crystallized  rocks.  It  there- 
fore follows  that  pressure  is  one  of  the  potent  forces  which  result  in  the 
devitrification  of  glass.  In  general  it  may  be  said  that  rocks  near  the 
surface,  whether  original  magmas,  sediments,  or  schists  and  gneisses  partly 
altered  in  the  belt  of  weathering,  very  frequently  contain  amorphous  prod- 
ucts; whereas  rocks  which  have  been  altered  while  deeply  buried  rarely 
contain  any  considerable  quantity  of  amorphous  material.  It  is  believed 
that  the  explanation  of  the  difference  is  largely  due  to  difference  in  pres- 
sure. At  depth  where  pressure  is  forceful  the  amorphous  products  which 
occupy  more  space  than  their  crystallized  equivalents  either  have  not 
formed  or  if  formed  at  the  surface  and  deeply  buried  have  become  crystal- 
lized, the  pressure  being  one  of  the  important  forces  in  the  process. 

Recrystallization  and  condensation  of  crystallized  compounds. PreSSUl'e  ma}7  llldllCe  cliem- 

ical  action  upon  crystallized  compounds,  producing  recrystallized  products 
of  a  different  kind  and  with  more  compact  molecules,  and  therefore  of 
greater  specific  gravity.  In  some  cases  the  recrystallization  has  occurred 

f'Tolman,  C.  F.,  jr.,  Professor  Spring  on  the  physics  and  chemistry  of  solids:  Jour.  Geol.,  vol.  6, 
1898,  p.  320. 

6 See  Dana,  J.  D.,  Manual  of  geology,  American  Book  Co.,  4th  ed.,  1895,  p.  265. 


104  A  TREATISE  ON  METAMORPHISM. 

at  least  twice.  After  one  set  of  compounds  was  produced  recrystallization 
again  occurred,  producing  heavier  compounds.  The  first  change  may  be 
illustrated  by  the  rearrangement  of  minerals  which  constitute  mud  so  as  to 
produce  mica,  quartz,  and  feldspar,  and  the  second  stage  may  be  illustrated 
by  the  development  from  the  latter  rock  of  the  still  heavier  minerals,  garnet, 
stjuirolite,  etc,  (See  p.  685.)  However,  the  process  of  recrystallizatioii 
in  nature  works  in  connection  with  more  rapid  solution  of  minerals  where 
strained  (see  pp.  95-98)  and  with  other  forces.  Its  full  consideration  is 
therefore  deferred  to  Chapter  VIII  (pp.  686-698). 


GENERAL    STATEMENTS. 


Where  pressure  is  unimportant,  as  near  the  surface  of  the  earth,  the 
chemical  reactions  are  ordinarily  controlled  by  other  factors  than  pressure; 
but  as  the  pressure  increases,  due  to  depth  below  the  surface  or  other  causes, 
it  becomes  a  more  and  more  important  factor  in  the  reactions  which  occur. 
But  it  is  shown  in  Chapter  VI,  on  "  Weathering,"  that  pressure  may  be  an 
important  factor  in  chemical  reactions  comparatively  near  the  surface.  This 
is  illustrated  by  granitic  rocks  in  the  District  of  Columbia,  described  by 
Merrill,"  which  when  brought  to  the  surface  underwent  rapid  disintegration, 
hydration,  and  expansion.  The  pressure  of  a  few  feet  of  rock  was  appa- 
rently sufficient  to  prevent  the  completion  of  these  reactions,  and  thus  it  is 
clear  that  the  adjustment  between  chemical  reaction  and  pressure  may  be 
very  delicate. 

However,  as  explained  in  Chapter  VI,  chemical  reactions  near  the  sur- 
face do  extensively  take  place  with  expansion  of  volume,  and  therefore  in 
spite  of  some  pressure.  But  it  is  also  shown  (Chapter  VIII)  that  the 
pressure  becomes  a  more  and  more  potent  factor  in  controlling  the  reac- 
tions; and,  finally,  that  there  exists  a  lower  zone  of  anamorphism  in  which 
this  is  the  dominant  force.  In  this  zone  the  chemical  changes  so  take  place 
as  to  lessen  the  volume  of  the  compounds,  and  therefore  to  produce  heavy 
minerals.  Moreover,  the  reactions  which  occur  in  this  lower  zone  are 
frequently  just  the  reverse  of  those  which  take  place  in  the  upper  zone, 
where  pressure  is  una"ble  to  control,  or  the  reactions  in  the  two  zones 
reverse  each  other. 


"Merrill,  G.  P.,  Disintegration  of  the  granitic  rocks  of  the. District  of  Columbia:  Bull.  Geol.  Soc. 
America,  vol.  6,  1895,  pp.  322-332. 


THE  HEAT  OF  SOLUTION.  105 

HEAT. 

Heat  is  a  very  important  factor  in  chemical  action.  In  the  heat  factor 
two  points  are  involved:  first,  the  general  effect  of  heat;  and,  second,  the 
effect  of  change  in  temperature  in  consequence  of  the  reactions. 

As  to  the  first  of  these,  in  the  lithosphere  the  higher  the  temperature  in 
general  the  more  rapid  the  alteration.  To  this  law  there  may  be  excep- 
tions, but  none  are  positively  known  to  me. 

As  to  the  second  point,  the  chemical  effect  due  to  the  change  in  tem- 
perature in  consequence  of  a  reaction  is  much  more  complicated. 

In  considering  whether  heat  be  liberated  or  absorbed  as  a  result  of  a 
chemical  reaction  it  is  necessary  to  take  into  account  the  heat  changes  in 
solution,  the  heat  changes  in  precipitation,  the  heat  changes  in  mixing 
solutions,  and  the  heat  effects  of  chemical  reactions. 

"By  the  heat  of  solution  is  meant  the  quantity  of  heat  produced  by 
the  solution  of  1  gram  molecule  of  a  substance  in  a  large  quantity  of  the 
solvent."11  It  has  already  been  seen  that  in  general  the  volume  of  the 
solvent  and  salt  is  greater  than  that  of  the  solution,  and  that  in  this  case 
there  is  usually  liberation  of  heat  and  consequently  rise  in  temperature; 
but  in  exceptional  cases  the  volume  of  the  salt  and  solvent  is  less  than  that 
of  the  solution,  and  in  this  case  there  is  generally  absorption  of  heat,  and 
consequent  fall  in  temperature.  The  total  effect  as  to  the  liberation  or 
absorption  of  heat  depends  upon  whether  the  total  of  the  factors,  change 
in  volume,  change  of  the  solid  to  its  dispersed  form  in  the  solution,  and 
the  heat  factor  of  dissociation,  provided  this  occurs,  is  plus  or  minus. 
Decrease  of  volume  tends  to  liberate  heat;  increase  of  volume  tends  to 
absorb  heat.  The  change  from  the  solid  to  the  dispersed  state  of  solution 
absorbs  heat.  The  supposed  dissociation  of  a  substance  into  its  ions  is 
regarded  as  attended  with  either  a  liberation  or  an  absorption  of  heat, 
though  liberation  is  held  to  occur  more  frequently.'' 

In  precipitation  the  heat  effect  is  just  the  opposite  from  that  of  solution 
and  is  equivalent  to  the  heat  effect  of  the  solution  of  an  equal  amount  of 
the  like  salt.  "In  general,  in  comparing  substances  which  are  chemically 
analogous  and  soluble  with  difficulty,  the  heat  of  precipitation  (—the 
negative  value  of  the  heat  of  solution)  is  the  greater  the  more  insoluble 

"Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London, 
1895,  p.  503. 

bNernst,  cit,  p.  562. 


106  A  TREATISE  ON  METAMORPHISM. 

the  substance  is."0  If  this  law  be  applicable  to  quartz  and  to  silicates  it  is 
of  great  importance  in  metamorphism,  because  these  are  the  substances 
most  largely  dissolved  and  deposited  by  the  ground  water,  with  the  pos- 
sible exception  of  the  carbonates. 

As  to  the  heat  relations  when  two  solutions  are  mixed,  Ostwald  states 
that  in  mixing  solutions  heat  is  produced  by  the  work  between  the  hetero- 
geneous molecules,  and  heat  is  used  in  separating  and  spreading  out  the 
homogeneous  molecules.  The  sum  of  these  may  be  positive  or  negative, 
but  in  most  cases  the  former  is  the  case,  and  hence  the  two  liquids  usually 
become  warmer  when  they  are  mixed.6  Upon  the  same  point  Nernst  says: 
"No  heat  phenomena  result  from  the  mixture  of  salt  solutions  [provided 
that  no  precipitate  (and  no  volatile  compound)  is  produced]."0 

When  chemical  reactions  occur  there  is  a  certain  amount  of  heat  of 
formation  of  the  compounds.  "By  the  'heat  of  formation'  of  a  chemical 
compound  is  meant  the  quantity  of  heat  which  is  given  off  in  the  formation 
of  the  compound  from  its  respective  ingredients."'*  "The  'heat- toning'  of  a 
reaction  is  equal  to  the  sum  of  the  resulting  heats  of  formation  minus  the 
sum  of  the  heats  of  formation  of  the  vanished  molecules." d  In  whatever 
way  a  chemical  result  is  accomplished,  and  however  many  the  stages  of 
process  of  the  change,  "the  energy  differences  (and  therefore  the  heat  dif- 
ferences) between  two  identical  conditions  of  the  system  must  be  the  same, 
independently  of  the  way  by  which  the  system  is  transferred  from  one 
condition  to  the  other."' 

This  last  is  an  important  law  so  far  as  the  work  of  ground  waters  is 
concerned,  for  in  most  cases  we  know  only  the  opening  and  closing  stages 
of  the  processes  of  alteration,  and  can  ascertain  whether  the  heat  effect  of 
a  reaction  is  plus  or  minus  only  by  comparing  the  heat  required  for  the 
production  of  the  original  minerals  with  that  required  for  the  production  of 
the  secondary  minerals,  and  their  gaseous  and  fluid  by-products. 

While  the  above  gives  the  conclusions  as  to  the  heat  effect  of  indi- 
vidual reactions,  the  reaction  which  is  likely  but  not  certain  to  obtain  at 
moderate  pressure  and  temperature  is  covered  by  the  rule  of  Berthelot. 
He  says,  "Every  chemical  change  gives  rise  to  the  production  of  those 

«Nernst,  \V.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London  1895 
p.  504. 

»Ostwald,  W.,  Solutions,  translated  by  M.  M.  Pattison  Muir;  Longmans,  Green  &  Oo.  "New  York 
1891,  p.  308. 

cNernst,  cit.,  p.  508.  <*Nernst,  cit.,  p.  505.  «Nernst,  cit,  p.  496. 


CHEMICAL  REACTION  AND  HEAT.  107 

substances  which  occasion  the  greatest  development  of  heat."0  And  there- 
fore, "other  things  being  equal,  there  is  the  more  chance  that  a  substance 
can  be  formed,  the  greater  its  heat  of  condensation."6  While  these  are  the 
usual  rules,  they  are  not  broad  enough  to  cover  the  reactions  of  meta- 
morphism  under  all  pressures  and  temperatures.  A  more  general  statement 
of  the  law  as  to  the  relations  of  heat  and  chemical  reactions  is  that  of  vau't 
Hoff:  "On  the  whole,  the  preponderating  chemical  reactions  at  lower  tem- 
peratures are  the  combinings  (associations)  which  take  place  with  a  devel- 
opment of  heat,  while  the  reactions  preponderating  at  higher  temperatures 
are  the  cleavings  (dissociations)  which  take  place  with  the  absorption  of 
heat.""  The  meaning  of  this  law  may  be  illustrated  by  the  following 
reactions:  At  ordinary  temperatures  CO  combines  with  O,  producing  CO2, 
with  great  liberation  of  heat;  at  very  high  temperatures  CO2  dissociates 
into  CO  and  O,  with  very  great  absorption  of  heat.  This  illustration 
makes  it  clear,  as  stated  by  Nernst,  that  to  cover  all  cases  van't  Hoff's 
law  must  replace  that  of  Berthelot,  above  given.  Still  more  general  laws 
as  to  the  relations  of  heat  and  chemical  reactions  are  the  following:  "If 
we  heat  a  chemical  system,  at  constant  volume,  then  there  occurs  a 
displacement  of  the  state  of  equilibrium,  and  in  that  direction  towards 
which  the  reaction  advances  with  absorption  of  heat."d  "Those  chemical 
forces  which  condition  a  development  of  heat  will  always  be  weakened  by 
an  increase  of  temperature;  and,  conversely,  those  which  condition  an 
absorption  of  heat  will  be  strengthened  by  such  an  increase  in  tempera- 
ture; and  it  is  this  fact  which,  primarily,  gives  the  preceding  proposition 
its  universal  validity  ."'d  "If  we  heat  the  system,  therefore,  the  reaction 
which  takes  place  will  be  accompanied  by  absorption  of  heat;  if  we  cool 
the  system,  the  corresponding  reaction  will  develop. heat." e 

Now  that  the  general  laws  covering  the  mutual  influences  of  heat  and 
chemical  action  have  been  given,  we  may  consider  in  more  detail  their 
meaning.  The  speed  of  the  reaction  is  commonly  increased  much  more 
rapidly  than  the  increase  in  absolute  temperature.  Thus,  the  speed  of 
reaction  of  two  similar  solutions,  one  of  which  is  at  higher  temperature 

«  Nernst,  cit.,  p.  581,  quoting  Berthelot. 
6 Nernst,  cit,  pp.  585-586. 
e  Nernst,  cit.,  p.  583. 
a  Nernst,  cit.,  p.  566. 

«Ostwald,  W.,  Outlines  of  general  chemistry,  translated  by  James  Walker,  2d  ed.,  Macmillan  & 
Co.,  London,  1895,  p.  312. 


108  A  TREATISE  ON  METAMORPHISM. 

than  the  other,  may  be  far  greater  in  the  solution  at  high  temperature  than 
would  be  calculated  from  the  relative  absolute  temperatures  of  the  solutions. 

Indeed,  the  velocity  of  a  chemical  reaction  commonly  increases 
enormously  with  moderate  increase  of  temperature.  The  partial  explana- 
tion of  the  phenomena  lies  in  the  fact  that  in  most  cases  the  reactions 
themselves,  as  already  seen,  develop  heat,  which  immediately  reacts 
to  increase  the  kinetic  energy  of  the  remaining  molecules,  and  this  again 
increases  the  kinetic  energy  of  the  molecules,  and  so  on,  there  being 
continual  action  and  reaction  between  the  chemical  activity  and  the  rising 
temperature. 

Another  illustration  of  the  very  important  way  in  which  increase  of 
temperature  increases  chemical  action  is  the  increased  activity  of  substances 
which  at  low  temperatures  are  relatively  inert.  While  at  ordinary  tempera- 
tures carbon  dioxide  replaces  silica  in  silicates,  at  temperatures  of  100°  C. 
silica,  if  present  in  abundance,  may  replace  carbon  dioxide  in  carbonates." 
While  this  is  explained  in  part  by  the  increase  of  activity  of  silicic  acid 
with  increase  of  temperature,  it  doubtless  in  part  is  explained  by  the  law 
of  mass  action  and  the  increased  volatility  of  carbon  dioxide  at  higher 
temperatures. 

If  the  dissociation  theory  be  true,  a  third  factor  which  may  have  some 
effect  in  producing  speed  of  reaction  with  increase  of  temperature  is  the 
increase  in  the  amount  of  hydrolysis  with  increase  of  temperature.  This  is 
illustrated  by  ferric  chloride,  which  at  low  temperatures  is  regarded  as  but 
little  hydrolized,  but  at  high  temperatures  is  believed  to  be  hydrolized  to  a 
perceptible  extent  according  to  the  equation: 

Fe2Cl«+6H,O=Fe2(OH)6+6HCl. 

The  presence  of  the  feme  hydroxide  is  shown  by  the  color  of  the  solu- 
tion.6 In  a  similar  manner,  the  carbonates  and  silicates  are  believed  to  be 
hydrolized  to  a  much  greater  extent  at  high  temperatures  than  at  low 
temperatures.  This  is  illustrated  by  calcium  carbonate,  which  in  solution 
at  high  temperatures  gives  a  strong  alkaline  reaction  of  calcium  hydroxide, 
and  this  is  regarded  as  evidence  of  strong  hydrolysis.  It  is  possible  that 
hydrolysis  is  an  important  factor  in  the  reactions  which  take  place  in  the 
different  zones  of  metamorphism. 

"Bischof,  Gustav,  Elements  of  chemical  and  physical  geology,  translated  by  Paul  and  Drum- 
mond,  Harrison  &  Sons,  London,  vol.  1,  1854,  p.  6. 

*Ostwald,  W.,  Gmndlinien  der  anorganischen  Chemie,  Engelmann,  Leipzig,  1900,  p.  583. 


CHEMICAL  CHANGES  ACCELERATED  BY  HEAT.     109 

While  in  general,  speed  of  chemical  change  is  promoted  by  rise  of 
temperature,  as  indicated  by  the  second  part  of  van't  HofFs  law,  there  is  a 
limit  to  the  increase  of  speed  due  to  action  and  reaction  between  chemical 
change  and  heat,  for  when  the  temperature  becomes  too  high  a  reverse 
tendency  is  set  up,  since  the  compounds  formed  by  the  chemical  reactions 
frequently  can  not  exist  at  very  high  temperatures.  In  such  cases  the  rate 
of  reaction  may  cease  to  increase  with  increase  of  temperature,  and,  indeed, 
the  reactions  which  obtain  at  lower  temperatures  may  be  reversed. 

Just  as  a  slight  increase  of  temperature  may  enormously  increase  the 
speed  of  chemical  reactions,  so  a  slight  decrease  of  temperature  may  very 
greatly  lessen  the  speed  of  reactions.  Therefore,  if  the  reaction  be  one 
which  itself  absorbs  heat,  and  thus  lowers  the  temperature,  the  slight 
decrease  in  the  kinetic  energy  of  the  molecules  may  greatly  retard  the 
speed  of  the  reaction. 

At  the  very  moderate  temperatures  which  generally  prevail  within  the 
outer  part  of  the  crust  of  the  earth  the  heat  resulting  from  the  chemical 
changes  does  not  become  so  great  as  to  stay  the  reactions.  Therefore,  it 
may  be  said  that  the  chemical  reactions  which  take  place  with  liberation  of 
heat  promote  metamorphism,  and  those  which  take  place  with  absorption 
of  heat  retard  metamorphism.  The  great  importance  of  these  two  tenden 
cies,  as  applied  to  rocks,  will  be  shown  on  subsequent  pages  in  connection 
with  the  discussion  of  the  zones  of  katamorphism  and  anamorphism. 

On  subsequent  pages  it  will  be  seen  that  in  the  zone  of  katamorphism 
the  first  part  of  van't  Hoff's  law  or*the  rule  of  Berthelot  generally  prevails 
in  the  alterations  of  rocks  for  a  considerable  distance  from  the  surface. 
That  is  to  say,  on  the  whole  the  preponderating  chemical  reactions  are 
those  which  take  place  with  the  liberation  of  heat.  Moreover,  as  a 
consequence  of  increase  of  heat  with  depth,  at  a  very  moderate  depth 
the  temperature  is  rather  high.  Also,  igneous  rocks  give  high  temperatures 
to  the  surrounding  rocks  and  solutions.  As  a  result  of  any  of  these  causes, 
water  may  reach  the  moderate  temperature  of  100°  to  200°  C.,  and  such 
temperatures  increase  the  activity  of  water  in  an  amazing  degree.  (See 
pp.  79-81.)  Thus  we  see  that  in  the  zone  of  katamorphism  the  heat  of 
chemical  action,  and  that  derived  from  the  interior  of  the  earth  through 
conduction  and  convection  by  means  of  magma  and  water,  all  work 
together  to  increase  the  speed  of  chemical  action,  and  therefore  to  hasten 
metamorphism. 


110  A  TREATISE  ON  METAMORPHISM. 

However,  it  will  also  be  seeu  that  in  the  zone  of  anamorphism,  with 
pressure  as  a  dominant  factor,  reactions  very  generally  occur  with  the 
absorption  of  heat  under  the  second  part  of  van't  Hoff's  law.  Thus,  in  this 
zone  the  heat  effect  of 'the  chemical  reactions  is  to  stay  metamorpbism. 
But  while  the  reactions  which  occur  at  depth  are  very  generally  those 
which  absorb  heat,  it  must  be  remembered  that  in  the  zone  of  anamorphism 
the  amount  of  heat  available,  due  to  increase  of  heat  with  depth  and  to  the 
difficulty  with  which  the  heat  escapes  from  intrusive  rocks,  is  very  great. 
Therefore,  notwithstanding  the  fact  that  the  chemical  reactions  themselves 
absorb  heat,  the  temperature  is  much  higher  than  in  the  upper  zone. 
Consequently  one  would  expect  that  the  chemical  activity  would  be 
greater  in  the  zone  of  anamorphism  than  in  the  upper  zone  of  katamorphism; 
and  with  these  expectations  the  facts  correspond.  (See  pp.  660-661,  690- 
692,  749-751.) 

RELATIONS  OF  CHEMICAL  ACTION,  MECHANICAL  ACTION,  AND  HEAT. 

All  transformations  of  material  upon  the  earth,  provided  all  the  energy 
factors  be  taken  into  account,  involve  the  expenditure  of  energy  and  the 
dissipation  of  part  of  it  as  heat.  If  this  were  not  true  it  would  be  possible 
to  manufacture  an  engine  by  means  of  which  an  equal  or  greater  amount 
of  energy  is  available  for  work  than  is  expended  in  driving  the  engine,  and 
perpetual  motion  would  be  possible.  In  metamorphism  of  rocks,  in  order 
that  the  above  general  statement  as  to  the  expenditure  of  energy  shall  be 
true,  it  is  necessary  to  take  into  account  the  chemical  force,  mechanical 
force,  and  heat  which  promote  the  transformations.  In  those  cases  where  a 
transformation  of  material  does  not  at  first  sight  appear  to  demand  the 
expenditure  and  dissipation  of  energy,  this  is  due  to  the  fact  that  some  of 
the  energy  factors  are  overlooked. 

It  has  been  noted  that  chemical  actions  are  reversible,  and  it  will  be 
seen  subsequently  that  chemical  reactions  which  take  place  on  a  large  scale 
in  the  zone  of  katamorphism  are  reversed  in  the  zone  of  anamorphism. 
When  a  chemical  reaction  takes  place,  and  laier  that  reaction  is  reversed 
and  the  cycle  is  repeated,  exterior  energy  must  have  been  expended  and 
dissipated.  To  illustrate,  let  us  consider  the  reversible  reaction 

FeA+.SHsO^FeA.  3H2O. 

The    reaction    may    advance    from   left   to    right    by    the    expenditure    of 
chemical  energy  alone,  and  as  a  result  of  the  process  heat  is  liberated. 


CHEMICAL  ACTION,  MECHANICAL  ACTION,  AND  HEAT.       Ill 

However,  to  reverse  the  reaction  or  to  advance  it  from  right  to  left 
requires  the  expenditure  of  a  greater  amount  of  external  energy  than  the 
chemical  energy  expended  in  the  first  reaction.  In  the  reaction  given 
the  available  external  energy  may  be  from  one  of  two  sources — heat  or 
mechanical  action.  The  ferric  hydrate  may  be  broken  into  ferric  oxide 
and  water  by  heating.  Also,  if  the  pressure  be  very  great  and  water  have 
a  chance  to  escape  the  same  transformation  may  take  place  by  the 
expenditure  of  mechanical  energy.  Doubtless  in  nature  in  many  cases 
both  of  these  forces  unite  in  the  process,  but  whether  the  dehydration  takes 
place  as  a  result  of  the  expenditure  of  heat  energy  alone  or  mechanical 
energy  alone,  or  the  two  combined,  a  greater  amount  of  energy  must  be 
expended  than  the  cnemical  energy  expended  in  the  hydration  of  the  iron. 
Hence,  when  hydration  takes  place  in  the  zone  of  katamorphism  energy  is 
expended.  This  is  potential  chemical  energy.  When  dehydration  takes 
place  in  the  zone  of  anamorphism,  reversing  the  first  process,  energy  is  also 
expended.  This  is  either  potential  mechanical  energy  or  the  energy  of 
heat,  or  the  two  together.  I  say  potential  mechanical  energy,  for  I  have 
held  in  another  place  that  all  earth  movements,  provided  all  the  factors  are 
taken  into  account,  result  in  bringing  the  material  moved  nearer  the  center 
of  the  earth,  and  therefore  the  energy  expended  is  the  potential  gravitative 
energy  of  position." 

The  reasoning  applied  to  the  case  of  hydration  and  dehydration  of 
ferric  oxide  is  applicable  to  every  other  reversible  reaction  in  metamor- 
phism;  hence,  when  we  take  all  the  energy  factors  into  account,  at  the  end 
of  the  process  energy  has  been  expended.  Furthermore,  a  part  of  this 
energy  during  the  process  has  been  transformed  to  heat  and  dissipated;  for 
in  all  transformations  of  energy  there  is  an  inevitable  tendency  for  some 
of  the  energy  to  run  down  into  the  lowest  form,  heat,  a  portion  of  which 
is  lost.6 

In  conclusion,  therefore,  in  the  zone  of  katamorphism,  while  chemical 
reactions  frequently  take  place  which  liberate  heat  and  expand  the  volume, 
and  in  the  zone  of  anamorphism  chemical  reactions  take  place  which  absorb 
heat  and  condense  the  volume,  in  both  zones  alike  when  all  of  the  energy 

« Van  Hise,  C.  R.,  Earth  movements:  Trans.  Wisconsin  Acad.  Sci.,  Arts,  and  Letters,  vol.  11, 
1898,  pp.  487,  488,  512-514. 

h  Daniell,  Alfred,  A  text-book  of  the  principles  of  physics,  3d  ed.,  Macmillan  Co.,  New  York, 
1895,  p.  51. 


112  A  TREATISE  ON  METAMORPHISM. 

factors  are  taken  into  account  the  reactions  take  place  in  such  a  way  as  to 
demand  the  expenditure  of  energy  and  the  loss  of  a  part  of  it. 

Where  chemical  force,  mechanical  force,  and  high  temperature  work 
together,  with  an  abundance  of  water,  as  an  agent  of  metamorphism,  the 
speed  of  rock  metamorphism  is  very  great  .as  compared  with  the  slow 
alterations  which  occur  at  the  surface  of  the  earth.  For  instance,  Barus 
finds  that  water  at  temperatures  above  185°  C.  and  under  high  pressure  is 
capable  of  very  rapidly  uniting  with  glass,  forming  a  new  compound,  which 
at  these  temperatures  is  liquid,  and  which  he  calls  water  glass.  In  a  retort 
he  combined  210  grams  of  glass  and  50  grams  of  water  in  twelve  hours 
at  a  temperature  of  210°  C.  into  water  glass,  which  was  liquid  at  that 
temperature,  but  became  a  clear  solid  at  ordinary  temperatures." 

Not  only  amorphous  compounds  but  crystalline  minerals  also  are  acted 
upon  rapidly  at  such  temperatures  and  pressures.  At  180°  C.,  with 
pressure  sufficient  to  keep  the  water  in  the  liquid  form,  Lemberg6  has 
completely  dissolved  zeolites  in  pure  water.  Under  similar  conditions 
it  has  also  been  shown  that  pure  water  acts  rapidly  upon  powdered 
anhydrous  silicates.  For  instance,  Forchhammer  showed  that  water  under 
these  conditions  dissolves  potassium  silicate  from  powdered  orthocla,se.c 

Within  the  zone  of  rock  flowage  temperatures  and  pressures  higher 
than  those  with  which  these  experiments  have  been  made  are  available,  and 
it  is  therefore  to  be  supposed  that  in  the  zone  of  anamorphism  there  is  rapid 
transformation  of  the  minerals  to  forms  which  are  relatively  stable  under 
the  conditions  obtaining  at  any  given  time  and  place.  So  far  as  substances 
have  not  a  compact  state  of  aggregation  energy  is  potentialized.  Pressure 
being  a  very  potent  factor,  the  transformations  would  of  course  be  into 
condensed  systems,  or  into  minerals  having  high  specific  gravity  and 
probably  complex  molecular  structure.  It  is  evident  that  in  the  forces  of 
chemical  action,  mechanical  action,  and  heat,  and  the  agent,  water,  we  have 
adequate  causes  for  the  crystallization  of  amorphous  compounds,  for  the 
recrystallization  of  strained  minerals,  and  for  the  recrystallization  of  highly 

«  Barus,  C.,  The  compressibility  of  liquids:  Bull.  U.  S.  Geol.  Survey  No.  92,  1892,  pp.  78-84. 
Hot  water  and  soft  glass  in  their  thermo-dynamic  relations:  Am.  Jour.  Sci.,  4th  ser.,  vol.  9,  1900, 
pp.  164-65. 

&  Doelter,  C.,  Allgemeine  chemische  Mineralogie,  Wilhelm  Engelmann,  Leipzig,  1890,  p.  189. 

«  Forchhammer,  G.,  Ueber  die  Zusammensetzung  der  Porcellanerde  und  ihre  Entstehung  aus 
dem  Feldspath:  Poggendorff,  Annalen,  vol.  35,  1835,  p.  354. 


NATURE  OF  PRECIPITATION.  113 

potentialized  minerals  to  lower  potentialized  forms.  These  changes  are 
illustrated  by  the  passage  of  glass  to  a  crystalline  form,  by  the  passage  of 
minerals  from  a  strained  to  an  unstrained  condition,  and  by  the  passage 
of  minerals  of  low  specific  gravity  to  minerals  of  higher  specific  gravity. 

PRECIl'ITA  TION. 

From  solutions,  by  changing  conditions,  solids  may  separate.  This 
process  is  called  precipitation.  Since  precipitation  from  ground  water 
solutions  is  of  the  utmost  importance  in  metamorphism,  it  is  necessary  to 
consider  fully  the  conditions  under  which  precipitation  takes  place.  It  has 
already  been  seen  that  in  solutions  the  ingredient  which  is  present  in  excess 
is  called  the  solvent  and  the  ingredients  which  are  subordinate  are  the  sub- 
stances dissolved.  When  from  solutions  the  substance  in  excess,  or  the 
solvent,  separates,  this  is  called  a  freezing  of  the  solution.  When  in  the 
solution  the  substances  dissolved  first  separate,  this  is  called  crystallizing 
out  of  the  materials  dissolved.  "The  processes  of  freezing  and  of  crystal- 
lizing out  are  both  to  be  considered  from  the  same  point  of  view ;  and  when 
we  are  not  dealing  with  dilute  solutions  where  one  ingredient  is  present  in 
large  excess,  but  with  a  mixture,  where  both  ingredients  are  present  in 
about  the  same  proportions,  then  we  would  be  in  actual  doubt  whether  the 
separation  should  be  regarded  as  a  freezing  (of  the  solvent)  or  a  crystal- 
lizing out  (of  the  substance  dissolved),  or  perchance  of  both  processes."" 

The  necessary  condition  for  precipitation  is  supersaturation;  for  if  a 
solution  be  not  saturated  it  will  take  more  material  into  solution ;  but  if  a 
solution  be  sufficiently  supersaturated  some  of  the  material  must  be  thrown 
down  or  be  precipitated.  If  solids  are  present  similar  to  the  compounds 
in  solution,  considerable  supersaturation  does  not  occur.  This  is  very 
frequently  the  case  with  ground  solutions.  Under  such  circumstances 
the  salts  in  solution  separate  out  upon  the  minerals  already  present,  or  the 
minerals  grow.  At  any  given  pressure  and  temperature,  provided  the 
changes  occur  slowly,  equilibrium  is  nearly  retained  by  this  continuous 
adjustment.  This  relation  between  minerals  already  present  and  solutions 
is  one  of  the  most  important  factors  which  control  the  growth  of  minerals 
which  are  present.  If,  for  instance,  in  a  complex  solution  containing 

"Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,   Macmillan  Co.,  New  York, 
1895,  p.  414. 

MON  XLVII — 04 8 


114  A  TREATISE  ON  METAMORPHISM. 

various  ions  there  are  also  various  crystalline  minerals,  the  moment  that 
the  solution  becomes  supersaturated  with  reference  to  several  ions  which 
may  unite  to  produce  one  of  the  solids  present,  this  union  will  take  place, 
the  material  will  be  precipitated  upon  the  minerals  of  that  kind,  and  thus 
they  will  grow. 

This  process  of  mineral  growth  applies  alike  to  minerals  in  magmas  and 
to  minerals  in  sedimentary  rocks.  If,  for  instance,  in  a  magma  plagioclase 
and  pyroxene  individuals  once  begin  to  form,  they  may  grow  to  large  size 
and  produce  a  gabbro.  In  a  sedimentary  rock  in  which  quartz  and  feld- 
spar particles  are  present  and  the  solutions  are  of  a  kind  which  furnish 
constituents  for  their  growth,  these  particles  are  likely  to  be  enlarged. 

Supersaturation,  and  consequently  precipitation,  may  result  in  various 
ways,  of  which  the  following  are  the  more  important:  (1)  Precipitation 
by  change  of  pressure,  (2)  precipitation  by  change  of  temperature,  (3) 
precipitation  by  reactions  between  aqueous  solutions,  (4)  precipitation 
by  reactions  between  aqueous  solutions  and  gases,  and  (5)  precipitation  by 
reactions  between  solutions  and  solids. 


I'RKCIPITATION  BY  CHAMJK  OF  PRESSl'UK. 


Change  of  pressure  may  result  in  supersaturatiou,  and  therefore  in 
precipitation.  Where  the  volume  of  the  solution  is  less  than  that  of  the 
solvent  and  substance  dissolved,  decrease  of  pressure  is  favorable  to 
precipitation.  Where  the  volume  of  the  solution  is  greater  than  that  of 
the  solvent  and  substance  dissolved,  increase  of  pressure  is  favorable  to 
precipitation.  The  volume  relations  are  opposite  in  the  cases  of  the 
crystallization  of  minerals  from  solutions  of  ground  water  and  the  crystal- 
lization of  minerals  from  magmas.  In  the  case  of  substances  dissolved  in 
ground  solutions  the  volumes  of  the  solutions  are  commonly  less  than  those 
of  the  solvent  and  the  substances  dissolved;  therefore  decrease  of  pressure 
is  favorable  to  precipitation.  But  in  the  case  of  crystallization  from 
magmas  the  volume  of  the  solution  is  greater  than  that  of  the  crystallized 
minerals;  therefore  pressure  is  favorable  to  crystallization. 

In  another  connection  it  is  suggested  that  under  certain  conditions  water 
and  magma  are  miscible  in  all  proportions.  (See  Chapter  VIII,  p.  723.) 
In  other  words,  there  is  every  gradation  from,  water  containing  compounds 
in  solution  to  magmas  containing  subordinate  amounts  of  water.  If  this 


LAWS  OF  PRECIPITATION.  115 

be  so,  ideally  there  must  be  a  neutral  point  in  which  the  volume  of  the 
material  is  the  same  whether  as  a  solution  or  as  a  solid.  In  this  case 
pressure  would  have  no  effect  upon  precipitation.  However,  the  precipita- 
tion of  any  part  of  the  material  from  a  solution  modifies  the  character  of 
the  remainder  of  the  solution,  and  it  is  not  to  be  supposed  that  a  case  is 
likely  to  occur  in  which  crystallization  of  material  takes  place  without  there 
being  any  pressure  effect. 

Where  circulating  waters  are  descending  the  pressure  is  increasing, 
and  where  ascending  the  pressure  is  decreasing.  Therefore,  in  the  case  of 
ordinary  ground-water  solutions  the  direction  of  water  circulation  which  is 
favorable  to  precipitation  is  ascension. 


PRECIPITATION  BY  (  II  \\(,l.  OF  TEMPERATI'KE. 


Change  of  temperature  may  result  in  supersaturation,  and  therefore  in 
precipitation.  In  general,  in  ground  solutions  increase  in  temperature 
increases  solubility.  (See  pp.  79-81.)  Therefore  decrease  in  temperature 
is  favorable  to  supersaturation  and  precipitation.  While  this  statement  is 
true  for  most  substances  at  temperatures  below  100°  C.,  and  is  correct  for 
many  substances  at  temperatures  considerably  higher  than  this,  at  very 
high  temperatures  the  conditions  are  reversed  for  some  substances.  (See 
p.  79.)  In  the  common  case,  that  of  precipitation  with  decrease  of 
temperature,  the  freezing  point  of  the  solution  is  lower  than  that  of  the 
solvent."  Apparently  the  amount  of  lowering  is  proportional  to  the 
molecular  weights,  and  is  stated  by  Raoult  as  follows:  "One  molecule  of 
any  compound  when  dissolved  in  100  molecules  of  a  liquid  lowers  the 
solidification  point  of  the  liquid  by  an  amount  which  is  nearly  constant, 
viz,  0.62°;"  or,  the  molecular  depression,  when  the  solvent  is  to  the 
solute  as  1:100,  is  0.620.6  In  dilute  solutions  "of  salts  in  water,  the 
molecular  depression  may  be  larger  than  this,  in  which  case  the  substance 
is  regarded  by  many  as  dissociated."  It  was  by  the  application  of  the 
principle  of  molecular  depression  that  Kahlenberg  and  Lincoln  were  able 
to  reach  the  conclusion  already  given  (p.  87),  that  silica  goes  into 
solution  as  colloidal  silicic  acid.  When  the  silicates  are  dissolved  and 


«Ostwald,  W.,  Solutions,  translated   by  M.  M.   Pattison  Muir;  Longmans,  Green   &  Co.,  New 
York,  1891,  p.  199  et  geq. 

»Ost\vald,  Solutions,  tit.,  p.  208. 
rOstwald,  Solutions,  cit.  p.  214. 


116  A  TREATISE  ON  METAMOKPHISM. 

decomposed  by  hydrolysis  into  colloidal  silicic  acid  and  metallic  hydroxides, 
the  latter  (or,  according  to  the  dissociation  theory,  their  ions)  caused  the 
molecular  depression,  which  was  unaffected  by  the  colloidal  silicic  acid. 

In  precipitation  from  complex  mixtures  the  substances  do  not  solidify 
at  the  same  time.  The  compounds  crystallize  in  such  order,  and  the 
separated  solid  is  of  such  a  character,  that  "the  freezing  point  of  the 
remaining  liquid  is  lowered."  °  After  one  compound  has  separated  another 
follows,  which  again  lowers  the  freezing  point,  and  finally  a  liquid  is  left 
with  the  lowest  freezing  point,  and  this  liquid  is  the  last  compound  to 
crystallize. 

Change  in  temperature  is  the  rule  for  underground  circulating  waters. 
The  waters  which  are  passing  to  lower  levels  are,  on  the  average,  becoming 
warmer.'  Waters  which  are  rising  to  higher  levels  are,  on  the  average, 
becoming  colder.  Also  there  are  changes  of  temperature,  both  positive 
and  negative,  due  to  varying  local  conditions;  for  instance,  the  presence  of 
intruded  igneous  rocks.  Ascending  waters  are,  on  the  whole,  precipitating 
material,  because  they  are  losing  heat.  The  increase  in  the  capacity  to 
hold  material  in  solution  with  rising  temperature,  and  the  simply  enormous 
increase  in  this  capacity  as  the  temperature  becomes  very  high,  have 
already  been  pointed  out.  (See  pp.  79-81.)  During  the  upward  journey 
of  the  water  the  temperature  continuously  falls,  and  if  the  journey  be  long 
the  total  loss  of  heat  is  great,  and  the  amount  of  precipitation  is  correspond- 
ingly large.  Since  the  upward  course  of  the  water  is  likely  to  be  in  the 
larger  openings  (see  p.  583).  such  as  the  spaces  of  porous  sandstones, 
faults,  joints,  etc.,  we  have  the  partial  explanation  of  the  filling  of  these 
openings  in  the  belt  of  cementation.  However,  this  general  statement 
needs  various  modifications,  dependent  upon  many  variable  factors.  (See 
pp.  629-640.) 

PHECIPITATIOX  BY  REACTIONS  BETWEEN  AQUEOUS  SOLUTIONS. 

It  has  already  been  seen  that  when  solutions  containing  various  salts 
are  mixed  the  resultant  solution  will  contain  all  the  salts  which  can  be 
made  by  the  various  combinations  of  their  positive  and  negative  ions. 
(See  p.  68.)  The  first  law  of  precipitation  may  be  stated  thus:  When  any 
combination  of  the  various  ions  in  a  solution  can  form  to  a  sufficient  extent 

"  Si-met,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  Co.,  New  York, 
1895,  p.  111. 


LAWS  OF  PRECIPITATION.  117 

to  be  insoluble  in  the  liquid  present,  such  compound  will  be  produced  and 
precipitated.     To  illustrate,  if  a  solution  of  BaCl2  be  added  to  a  solution 

if  Na^SOj,  the  ion  Ba  can  unite  with  the  ion  SO4  and  produce  the  insoluble 
compound  BaSO4,  which  will  be  precipitated. 

The  above  is  a  statement  of  the  empirical  facts.     The  explanation  of 

hese  facts  under  the  theory  of  dissociation  is  given  by  Ostwald  as  follows: 
In  any  given  case  there  is  a  constant  relation  between  the  amount  of  a 

•ompound  which  can  be  held  in  solution  and  the  number  of  free  ions  of 

hat  compound.     Upon  this  statement  are  based  the  laws  of  precipitation 

rom  solutions.     Says  Ostwald: 

In  solutions  a  state  of  equilibrium  subsists  between  the  ions  of  the  electrolyte 

ncl  the  nondissociated  portion.     To  take  the  simplest  possible  case,  if  we  have  a 

•inary  electrolyte  C,  which  can  break  up  into  ions  A  and  B',  and  if  a,  b,  and  c 

present  the  concentrations  of  these  three  constituents  in  a  given  solution,  then  the 

>1  lowing  simple  formula  holds  good:  ab=kc. 

Now,  the  two  kinds  of  ions  are  produced  in  equivalent  quantities,  in  the  above 
ase,  hence  a=b.  If,  further,  the  total  amount  of  the  electrolyte  =1,  and  a  repre- 

•nts  the  ionized  portion,  then  a=b=-  and  c=—  — ,v  being  the  volume  of  the  solu- 

v  v 

m  in  which  unit  quantity  (a  molecular  weight  in  grammes)  of  the  electrolyte  is 
•  mtained.     By  carrying  out  the  substitution  we  get  the  formula  -r- r=kv,  which 

xpresses  the  state  of  ionisation  of  an  electrolyte  at  the  dilution  v.a 

In  the  saturated  aqueous  solution  of  an  electrolyte  we  have  a  complex  equilib- 
ium.  On  the  one  hand  the  solid  is  in  equilibrium  with  the  nonionised  portion  of 
welf  which  is  in  solution,  while  on  the  other  hand  this  nonionised  portion  is  in 
•luilibrium  with  the  dissociated  part — i.  e.,  with  the  ions  of  the  same  substance, 
'he  first  equilibrium  comes  under  the  law  of  proportional  concentration,  or,  since 
v;  are  dealing  here  with  a  substance  of  unalterable  concentration  on  the  one  hand, 
te  concentration  of  the  nonionised  portion  in  the  solution  must  have  a  perfectly 
cfinite  value.  For  the  second  equilibrium  we  have  in  the  simplest  case — i.  e.,  when 
te  ions  of  the  compound  are  monovalent — ab=kc,  a  and  b  representing  the  concen- 
rions  of  the  ions  and  c  the  concentration  of  the  nonionised  portion. 

Now,  since  c  is  constant  at  a  given  temperature,  as  we  have  already  seen,  kc, 
ad  therefore  ab,  must  be  constant  also.  Equilibrium  is  thus  established  between  a 
pecipitate  and  the  liquid  above  it  when  the  product  of  the  concentrations  of  the  two 
ins,  into  which  the  precipitate  falls,  has  a  definite  value.  This  product  may  be 
tcrned  the  solubility  product  for  the  sake  of  brevity. 

"  Ostwald,  W.,  Foundations  of  analytical  chemistry,  translated  by  George  McGowan,  Macmillan  & 
(  ,  London,  1895,  p.  59. 


118  A  TREATISE  ON  MET AMORPHISM. 

If  the  electrolyte  consists  of  polyvalent  ions  in  the  proportion  mA:nB,  the 
solubility  product  takes  the  form:  ambn=  constant.0 

From  the  foregoing  follows  Ostwald's  statement  of  the  first  law  of  pre- 
cipitation, already  given:  "Whenever  in  any  liquid  the  solubility  product 
of  a  solid  is  exceeded,  the  liquid  is  supersaturated  with  respect  to  that  solid,'" 
and  therefore  precipitation  of  the  salt  follows.  Of  the  various  salts  which 
may  be  precipitated  from  a  solution,  that  one  will  be  precipitated  first 
whose  solubility  product  exceeds  its  constant  of  solubility. 

Ostwald  illustrates  this  by  the  cases  already  cited:  If  a  solution  of 
BaCl2  be  added  to  Na2S04,  BaSO4  will  be  precipitated.  According  to 
Ostwald's  view,  this  happens  because  the  solubility  product  of  the  ions  in 
BaSO4  is  very  small. 

The  second  law  of  precipitation  follows  from  the  fact  that  "the  solu- 
bility of  one  salt  is  depressed  in  the  presence  of  another  having  a  common 
ion.""  This  is  equivalent  to  saying  that  "the  solubility  of  each  molecular 
species  in  a  mixture  is  always  smaller  than  for  the  particular  species  when 
alone.'"1  Hence,  when  to  a  solution  containing  certain  ions  a  solution  is 
added  which  has  an  ion  in  common  with  one  of  those  already  in  the  solu- 
tion, supersaturation  and  precipitation  are  promoted.  An  example  of  this 
is  the  addition  of  HC1  to  a  solution  of  BaCl2.  The  chlorine  ion  is  common, 
and  if  the  solution  is  near  saturation  before  the  HC1  is  added,  BaCl2  will  be 
precipitated.  Again,  if  one  adds  a  saturated  solution  of  NaC103  to  a  satu- 
rated solution  of  KC1O3,  an  abundant  precipitate  of  the  latter  salt  will  form. 

The  above  law  is  a  general  statement  which  includes  the  rule  that  "The 
addition  to  a  solution  of  a  liquid  which  is  able  to  form  a  homogeneous  whole 
with  the  solution  causes  precipitation  of  more  or  less  of  the  substance  in 
solution  if  that  substance  is  insoluble  in  the  liquid  which  is  added."6  This 
rule  is  illustrated  by  the  same  examples.  It  follows  from  this  that  "in 
order  to  precipitate  a  substance  completely  from  its  solution,  an  addition  of 
an  excess  of  the  precipitant  is  an  advantage."7 

The  converse  of  the  second  law  of  precipitation  is:  The  solubility  of  a 
salt  increases  on  the  addition  of  a  second  salt  containing  no  ion  in  common. 

a  Ostwald,  W.,  Foundations  of  analytical  chemistry,  translated  by  George  McGowan,  Macmillan 
&  Co.,  London,  1895,  p.  76. 

*  Ostwald,  Foundations,  cit.,  pp.  76-77. 

"Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  Co.,  New  York, 
1895,  p.  446. 

<*Nernst,  cit,  p.  453. 

'  Ostwald,  Solutions,  p.  90. 

/Xernst,  cit.,  p.  449. 


LAWS  OF  PRECIPITATION.  119 

To  illustrate:  "If  one  adds  some  KXO3  to  AgBr03,  a  number  of  molecules 
of  AgNO3  and  also  of  KBr03  will  be  formed.  This  will  result  in  a  diminution 
of  the  number  of  the  molecules  of  AgBrO3,  which  must  be  replaced  from 
the  solid  salt,"  or  the  solubility  will  be  increased." 

Cameron  gives  two  illustrations  of  this  converse  which  are  of  great 
importance  in  ground  solutions  : 

Gypsum,  which  is  essentially  the  salt  calcium  sulphate  containing  some  water, 
is  sparingly  soluble  in  water.  But  the  addition  of  an  electrolyte  with  no  common 
ion,  such  as  sodium  chloride,  will  considerably  increase  the  solubility  of  the  gypsum. 
Some  experiments  made  in  this  laboratory  have  shown  that  in  moderatelv  strong 
brines  containing  only  sodium  chloride  gypsum  can  be  regarded  as  a  soluble  salt. 
The  reason  for  this  is  readily  seen  when  the  substances  which  are  formed  are  con- 
sidered, both  the  calcium  chloride  and  the  sodium  sulphate  being  very  soluble  salts. 
The  transportation  of  large  quantities  of  lime  by  the  drainage  and  ground  waters  in 
arid  regions  where  these  salts  arc  found  is  readily  explicable  from  this  point  of  view. 

Calcium  carbonate,  so  abundant  and  so  important  in  nature,  is  dissolved  in  a 
precisely  similar  way;  but  the  ionization  of  carbonates  being  relatively  small,  the 
effect  is  not  so  striking  and  relatively  much  less  lime  is  transported  in  the  solution. 
Treadwell  and  Reuter*  have  recently  published  investigations  on  this  point  and  find 
the  solubilit3r  of  calcium  carbonate  in  sodium  chloride  solutions  does  not  become 
markedly  large  until  considerable  concentrations  of  the  latter  salt  are  reached.  The 
effect  of  carbon  dioxide  in  forming  the  more  soluble  bicarbonate  of  lime  undoubtedly 
is  an  important  element  in  this  connection,  but  as  the  ionization  is  but  little  affected 
by  its  presence  its  influence  must  be  small  in  the  presence  of  such  a  salt  as  sodium 
chloride/ 

PRECIPITATION  BY  REACTIONS  BETWEEN  AiJUEOtS  SOLUTIONS  AND  GASES. 

Another  case  of  precipitation  occurring  in  nature  follows  as  a  result 
of  mixing  solutions,  one  of  which  is  a  gas  which  acts  upon  the  compounds 
in  the  aqueous  solution,  producing  ions  of  a  different  kind  from  those  before 
present,  and  in  some  cases  forming  compounds,  the  solubility  of  which  is 
so  small  that  precipitation  results.  Perhaps  the  most  important  case  of 
this  kind  is  the  mixing  of  oxygen  with  a  solution  containing  salts  of  iron 
protoxide.  As  a  result  of  this  the  iron  is  changed  from  ferrous  to  ferric 
form,  and  the  latter  is  precipitated  as  a  sesquioxide  or  hydrosesquioxide  of 
iron.  In  the  latter  case  hydration  occurs  simultaneously  with  oxidation. 

«Nerast,  cit.,  p.  450. 

6  Treadwell,  F.  P.,  and  Reuter,  M.,  Ueber  die  Loslichkeit  der  Bikarbonate  des  Calciums  und 
Magnesiums:  Zeitschr.  fiir  anorgan.  Chemie,  vol.  17,  1898,  p.  170. 

c  Cameron,  F.  K.,  Application  of  the  theory  of  solutions  to  the  study  of  soils:  Kept.  No.  64,  Field 
Operations  of  Division  of  Soils,  1899,  U.  S.  Dept.  of  Agric.,  1900,  pp.  150-151. 


120  A  TREATISE  ON  METAMORPHISM. 

i  t 

PRECIPITATION  BT  REACTIONS  BETWEEN  SOLUTIONS  AND  SOLIDS. 

"If  one  pours  a  solution  of  KBr  over  solid  AgCl,  .  .  .  the  bromine 
existing  in  the  solution  will  be  largely  replaced  by  chlorine,  because  as 
AgBr  is  much  less  soluble  than  AgCl  an  equivalent  quantity  of  AgCl  will 
be  changed  into  AgBr.  This  is  also  established  by  experiment.  If  one 
knows  the  solubilities  of  AgCl  and  AgBr,  then  for  a  given  concentration  of 
KBr  we  may  state  the  point  of  equilibrium  which  the  system  strives  to 
reach.""  Hence  we  conclude  that  if  a  salt,  A,  is  treated  with  a  saturated 
solution  of  another  salt,  B,  a  greater  or  less  part  of  the  salt  B  may  separate 
out,  the  salt  A  being  taken  into  solution  at  the  same  time.  In  this  case 
"the  active  mass  of  the  solid  substance  is  a  constant."  The  meaning  of 
this  is  that  if  any  of  a  solid  salt  is  present  after  the  reaction  has  ceased 
there  was  sufficient  to  produce  equilibrium  between  the  salt  and  the 
solution. 

An  excellent  case  illustrating  precipitation  from  solution  in  nature  by 
the  action  of  a  solid,  one  of  the  most  fundamental  importance,  is  the  partial 
dolomitization  of  the  calcium  carbonate  of  shells  and  corals  by  the  sea 
waters,  which  contain  both  calcium  and  magnesium  salts.  In  this  case, 
under  the  law  of  chemical  equilibrium,  there  is  constant  action  and  reaction 
between  the  magnesium  salts  in  solution  and  the  solid  CaCO3.  The 
magnesium  and  calcium  partially  interchange,  the  calcium  going  into 
solution  by  uniting  with  the  ions  before  combined  with  the  magnesium,  and 
the  magnesium  simultaneously  uniting  with  the  C03  ion  before  united  with 
the  calcium  and  thus  being  thrown  down  as  MgC03.  Thus  the  calcite  is 
partially  dolomitized. 

This  case  of  dolomitization  well  illustrates  the  principle  that  simul- 
taneously with  the  precipitation  of  one  element  or  mineral  another  element 
or  mineral  may  be  dissolved,  one  being  conditioned  upon  the  other.  There 
are  very  numerous  complicated  cases  of  this  kind  which  need  investigation. 
(See  pp.  203-206.) 

The  solids  present  exert  an  important  influence  in  precipitation 
independently  of  the  passage  of  elements  of  the  solids  into  the  solutions. 
That  is  to  say,  if  there  be  solids  present,  even  if  none  of  the  elements  of 
any  of  such  compounds  pass  into  solution,  these  solids  may  influence  the 

"Nernst,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  Co.,  New  York,  1895, 
p.  452. 

''Nernst,  cit.,  p.  450. 


LAWS  OF  PRECIPITATION.  121 

nature  of  the  precipitation.  This  statement  is  applicable  both  to  com- 
pounds present  in  solutions  before  precipitation  begins  and  to  compounds 
formed  by  precipitation  itself.  Once  any  precipitate  begins  to  form,  par- 
ticles of  that  precipitate  are  present  and  influence  further  precipitation, 
precisely  as  do  other  solids  which  were  present  before  the  precipitation 
began.  .The  proof  of  the  influence  of  the  solids  present  is  furnished  by 
the  very  well-known  tendency  to  the  enlargement  of  mineral  particles 
already  existing  in  preference  to  the  formation  of  new  individuals. 

The  growth  of  mineral  particles  already  present  is  probably  con- 
nected with  the  phenomenon  of  adsorption,  described  on  pages  64—65.  It  is 
there  noted  that  the  contact  film  of  solutions  with  solids  contains  more  than 
an  average  amount  of  material  in  solution.-  It  may  be  suggested  that  this 
is  due  to  the  molecular  attraction  of  the  crystal  for  the  molecules  in  solu- 
tion, just  as  the  adherent  film  of  the  liquid  itself  is  due  to  the  molecular 
attraction  between  the  solids  and  liquids.  As  the  particles  in  solution  move 
about  they  continually  impinge  against  the  solids  in  the  solutions.  These 
particles  thus  come  within  the  limits  of  the  molecular  attraction  of  the  solids 
and  are  to  a  certain  extent  held,  and  hence  the  concentration.  It  would 
follow  that  the  adherent  films  of  liquid  are  likely  to  become  supersaturated 
in  advance  of  the  remainder  of  the  solutions.  Under  these  circumstances  the 
moment  supersaturation  is  reached  with  reference  to  the  compounds  forming 
a  given  particle,  these  materials  will  be  deposited  upon  the  particle,  'and 
will  grow.  Precipitation  immediately  follows  supersaturation  of  the  con- 
centrated film  because  of  the  orienting  and  selecting  power  of  the  mineral 
particle  already  existing.  It  is  probable  in  the  case  of  a  given  mineral  that 
for  compounds  other  than  those  which  can  unite  to  produce  the  mineral 
supersaturation  can  take  place  to  some  extent,  and  that  from  this  slightly 
supersaturated  adherent  film  this  material  may  escape  into  the  free  solution. 
However,  when  such  solutions  become  supersaturated  in  the  presence  of  a 
mineral  which  could  use  them  they  would  be  thrown  down.  By  this  process 
is  explained  the  selective  power  by  which  each  mineral  particle  is  able  to 
take  from  solution  material  like  itself  and  add  it  to  itself;  and  also  the  fact 
that  particles  once  formed  abstract  materials  like  themselves  from  solutions 
in  preference  to  the  formation  of  new  particles."  The  presence  of  any 

aFor  explanation  of    adsorption    see    Ostwald,   W.,    Grundlinien  der  anorganischen  Chemie, 
Engelmann,  Leipzig,  1900,  pp.  387-389. 


122  A  TREATISE  ON  METAMORPHISM. 

mineral  species  will  prevent  considerable  supersaturation  of  the  solution,  so 
for  as  the  compounds  of  that  species  are  concerned.  The  result  is  that  if 
there  be  materials  in  a  solution  which  can  unite  to  produce  mineral  species 
which  are  present  they  will  do  so.  In  this  way  the  minerals  control  or 
guide  to  a  considerable  extent  the  character  of  the  solids  which  are  deposited, 
since  when  a  certain  mineral  is  absent,  before  that  mineral  can  begin  to  be 
precipitated  supersaturation  must  occur  with  reference  to  the  chemical 
combination  which  composes  it. 

Therefore  the  mineral  species  which  are  present  in  a  solution  have 
an  advantage  over  other  kinds  of  minerals  which  are  absent,  To  a  less 
degree,  minerals  which  are  abundantly  present  have  an  advantage  over 
those  which  are  sparse.  To  illustrate,  if  quartz  be  present  and  the  solu- 
tions contain  ions  of  silica,  it  will  be  apt  to  abstract  the  silica  from 
the  solutions  the  moment  supersaturation  occurs.  In  the  same  way,  if 
feldspar  be  present  and  there  are  ions  of  sodium,  calcium,  aluminum,  and 
silica  in  proper  proportions,  these  are  likely  to  be  grouped  together  to 
produce  feldspar.  Moreover,  it  appears  to  be  the  case  that  the  feldspar 
may  so  nearly  control  that  a  closely  analogous  feldspar  is  produced,  and 
twinning  and  other  phenomena  characteristic  of  the  original  grains  be 
continued  in  the  secondary  growth.  The  same  statements  apply  to 
hornblende,  tourmaline,  calcite,  and,  in  fact,  to  all  minerals  in  which  a 
secondary  growth  has  been  noted.  Of  course,  in  a  rock  in  which  there 
are  present  a  large  number  of  mineral  particles,  the  particular  mineral 
which  is  formed  will  depend  upon  the  various  ions  in  the  solution,  their 
relative  proportions,  and  the  relative  insolubility  of  the  salts.  For  instance, 
tourmaline  can  not  form  unless  the  boric  acid  ions  are  present;  horn- 
blende can  not  be  produced  unless  there  are  in  the  solution  all  the  bases 
demanded  by  that  mineral  in  sufficient  abundance.  Thus  the  particular 
mineral  which  forms  depends  upon  a  complicated  adjustment  of  the  mineral 
particles  present,  the  ions  present  in  the  solution,  their  relative  proportion, 
and  the  solubility  of  the  mineral  particles. 

In  the  above  chemical  principle  lies  a  partial  explanation  of  the  strange 
fact  that  minerals  are  so  firmly  cemented  by  material  like  the  dominant 
original  mineral.  In  quartzose  sandstone  the  chief  cement  is  silica;  in  feld- 
spathic  sandstone  the  chief  cement  is  likely  to  be  feldspar;  in  strongly  horn- 
blendic  rocks  one  of  the  chief  cements  is  hornblende,  and  so  on.  Another 


GROUND  WATER  UNIVERSAL.  123 

important  factor  in  the  process  is  the  extension  of  the  rock  masses  from 
the  places  of  solution  to  the  places  of  deposition.  For  instance,  in  any 
rock  which  extends  from  the  belt  of  weathering  to  the  belt  of  cementation 
the  water  at  the  places  of  solution  (especially  the  belt  of  weathering) 
would  obtain  material  adapted  to  the  enlargement  of  the  minerals  of  the 
same  rock  at  the  place  of  deposition  (especially  the  belt  of  cementation). 
Consequent  upon  the  two  factors  above  given,  rocks  in  many  instances 
are  cemented  by  minerals  like  those  present  before  cementation  began. 

SECTION  2.     CIRCULATION  AND  WORK  OF  GROUND  WATER. 
UNIVERSAL   PRESENCE    OF   WATER   IN    ROCKS. 

It  has  already  been  explained  at  the  opening  of  this  chapter  that  water 
is  the  great  dominating  agent  through  which  the  greatest  transformations 
are  accomplished.  Free  water  is  present  to  some  extent  in  all  rocks  within 
the  zone  of  observation.  That  it  is  abundant  in  porous  rocks  is  well 
known.  Water  has  also  the  power  to  slowly  penetrate  the  apparently  solid 
rocks.  Between  the  mineral  particles  there  is  space  sufficient  for  water  to 
make  its  way,  and  a  small  amount  of  water  is  found  in  the  most  massive 
and  relatively  impervious  rocks. 

Besides  the  free  water  in  rocks,  there  is  always  present  water  in  a 
combined  form.  The  combined  water  varies  from  a  small  fraction  of  1  per 
cent  to  several  per  cent.  Commonly  the  combined  water  does  not  fall 
below  0.50  per  cent,  and  seldom  is  higher  than  8  per  cent.  It  therefore 
appears  that  all  rocks  contain  water,  both  in  the  free  and  in  the  combined 
form.  The  amounts  of  each  of  these  are  very  variable.  Bischof  many 
years  ago  noted  the  penetration  of  basalt  by  water."  The  permeation  of 
apparently  solid  rocks  by  water  is  well  illustrated  by  the  readiness  with 
which  agate,  chalcedony,  and  such  materials  are  affected  by  a  staining 
solution.  When  agates  are  boiled  in  colored  solutions,  the  liquid  makes  its 
way  through  the  minute  subcapillary  spaces  so  small  that  the  microscope 
can  not  detect  them,  and  the  bands  are  differently  tinted,  the  amount 
of  deposited  coloring  material  depending  upon  the  relative  sizes  of  the 
minute  openings. 

°  Bischof,  Gustav,  Chemical  and  physical  geology,  translated  by  Paul  and  Drummond,  Harrison 
&  Sons,  London,  vol.  1,  1854,  p.  10. 


124  A  TREATISE  ON  METAMORPHISM. 

The  water  in  rocks  may  completely  or  partly  fill  the  openings.  Where 
the  openings  of  a  rock  are  completely  filled,  the  rock  is  saturated.  Unless 
all  the  openings  in  a  rock  are  subcapillary  it  will  remain  saturated  only 
so  long  as  it  is  surrounded  or  partly  surrounded  by  the  saturating  liquid. 
If  withdrawn  from  the  saturating  liquid,  all  the  water  may  be  drawn 
off  by  ordinary  physical  means  except  that  adhering  to  the  walls  of  the 
openings.  This  residual  amount  of  water  is  called  the  water  of  imbibition. 
The  difference  between  the  water  of  saturation  and  that  of  imbibition, 
which,  as  will  be  seen,  is  the  water  which  may  flow  somewhat  readily,  may 
be  called  the  water  of  hygrometricity.  In  the  rocks  having  subcapillary 
openings  (see  pp.  143-146)  the  attraction  extends  from  wall  to  wall,  and 
therefore  the  entire  film  of  water  in  the  spaces  adheres  to  the  rock  particles, 
or  is  water  of  imbibition.  In  the  rocks  having  subcapillary  pores  only,  the 
water  of  imbibition  and  saturation  is  the  same. 

The  next  question  which  arises  is  as  to  the  source  of  the  ground  water. 
On  pages  661-668  reasons  are  given  for  the  belief  that  the  circulation  in  the 
zone  of  anamorphism,  which  corresponds  to  the  zone  of  rock  flowage,  is 
very  slow  indeed.  In  this  deep-seated  zone  decarbonation,  dehydration, 
and  to  some  extent  deoxidation  of  the  rocks  take  place.  It  is  shown  (see 
pp.  764-766)  that  with  these  exceptions,  excluding  igneous  rocks,  the  compo- 
sition of  the  rocks  metamorphosed  in  the  zone  of  anamorphism  closely 
corresponds  with  their  original  composition,  contrasting  greatly  in  this 
respect  with  the  rocks  metamorphosed  in  the  zone  of  fracture.  From  these 
and  other  facts  it  is  certain  that  the  circulation  of  water  in  the  zone  of 
anamorphism  is  very  slow.  However,  it  is  probable  that  a  large  portion  of 
the  carbon  dioxide  and  water  liberated  slowly  makes  its  way  into  the  zone 
of  fracture.  It  is  also  explained  that  some  water  may  join  the  zone 
of  fracture  through  the  agency  of  igneous  rocks  which  enter  this  zone. 
But  the  amount  of  these  supplies  of  water  at  any  one  time  is  small — indeed, 
insignificant  compared  with  the  amount  required  to  keep  up  the  active 
circulation  which  we  know  exists  in  the  zone  of  fracture.  Since,  then,  it  can 
not  be  shown  that  any  considerable  fraction  of  the  water  of  circulation  of 
the  zone  of  fracture  is  derived  from  the  zone  of  rock  flowage,  we  can  only 
suppose  that  this  water  is  derived  from  precipitation.  The  subterranean 
water  is  therefore  predominantly  of  meteoric  origin. 


POKE  SPACE  IN  ROCKS.  125 

i 

POKE   SPACE   OF   ROCKS. 

The  pore  space  of  rocks  varies  from  a  small  fraction  of  1  per  cent  to  50 
per  cent,  or  more.  The  pore  space  in  compact,  strong,  igneous  rocks  is 
exceedingly  small.  For  instance,  in  fresh,  strong  granites  the  percentage 
of  water  absorbed  by  the  dry  rock  varies  from  0.08  to  0.20  per  cent,  which 
corresponds  to  a  pore  space  of  0.20  to  0.50  per  cent.  The  more  compact 
limestones  also  contain  very  little  pore  space.  Some  of  them  absorb  as 
smarll  an  amount  as  0.20  per  cent  by  weight  of  water,  which  corresponds  to 
a  pore  space  of  about  0.55  per  cent. 

Ordinary  compact  limestones  used  for  building  material,  when  satu- 
rated, contain  from  1  to  5  per  cent  of  water  by  weight,  and  this  corre- 
sponds to  a  pore  space  of  about  2.5  to  12.5  per  cent.  The  more  porous 
limestones  are  capable  of  absorbing  10  per  cent  or  more  of  water  by  weight. 
Sandstones  are  ordinarily  very  porous,  holding  from  about  2  or  3  to  15 
per  cent  of  water  by  weight.  This  corresponds  to  a  pore  space  of  from 
about  5  to  28  per  cent  Capacity  to  hold  about  10  per  cent  by  weight,  and 
therefore  a  pore  space  of  about  20  per  cent,  is  very  common  in  sandstones. 
The  extreme  of  porosity  for  sandstones  yet  reported  is  the  Dunnville  sand- 
stone of  Wisconsin,  which,  according  to  Buckley,  contains  a  fraction  more 
than  28  per  cent  of  air  space  when  dry,"  and  therefore  when  saturated  is 
capable  of  having  28  per  cent  of  its  volume  occupied  by  water.  According 
to  Merrill,6  chalk  may  contain  as  much  as  20  per  cent  by  weight  of  water. 
Supposing  the  specific  gravity  of  the  chalk  to  be  2.8,  this  corresponds  to  a 
pore  space  of  about  41  per  cent.  However,  in  coherent  rocks,  pore  spaces 
of  more  than  25  per  cent  are  rather  uncommon. 

In  unconsolidated  rocks  where  cementation  has  not  taken  place  at  all, 
and  in  products  of  the  belt  of  weathering,  the  pore  space  may  be  even 
greater  than  the  above  amounts.  If  grains  of  sand  are  spherical,  of  uniform 
size,  and  "  are  arranged  in  the  most  compact  manner  possible,  each  grain 
will  touch  the  surrounding  grains  at  twelve  points." c  In  this  case  the  pore 
space  will  be  25.95  per  cent.d  If  the  pai-ticles  be  spherical,  of  uniform 

"Buckley,  E.  K.,  Building  and  ornamental  stones  of  Wisconsin:  Bull.  Wisconsin  Geol.  and  Nat. 
Hist.  Survey,  No.  4,  1898,  p.  225. 

6 Merrill,  G.  P.,  Rocks,  rock- weathering,  and  soils,  Macmillan  Co.,  New  York,  1897,  p.  198. 

"Slichter,  C.  S.,  Theoretical  investigation  of  the  motion  of  ground  waters:  Nineteenth  Ann.  Kept. 
U.  S.  Geol.  Survey,  pt.  2,  1899,  p.  306. 

^Slichter,  cit.,  p.  310.  Becker,  G.  F.,  Geology  of  the  quicksilver  deposits  of  the  Pacific  coast: 
Mon.  U.  S.  Geol.  Survey,  vol.  13,  1888,  p.  399. 


126  A  TREATISE  ON  METAMORPHISM. 

size,  and  arranged  "so  that  the  lines  joining  their  centers  form  cubes,"0  this 
will  be  the  most  open  possible  arrangement,  In  this  case  the  pore  space 
will  be  47.64  per  cent.6 

King  has  made  a  number  of  experimental  determinations  of  the  pore 
space  of  unconsolidated  sands,  of  broken  rocks,  and  soils,  the  material  being 
packed  as  closely  as  he  was  able  to  pack  it."  Where  quartz  sand  com- 
prising materials  varying  greatly  in  coarseness  was  used,  a  pore  space  as 
low  as  25.43  per  cent  was  obtained/  But  "  well-rounded  grains  of  nearly 
uniform  diameter  tend  to  give  a  pore  space  which  lies  between  32  and  40 
per  cent.  *  *  *  For  simple  sands  with  angular  grains  the  pore  space  is 
much  larger  than  it  is  for  the  rounded  sands  of  the  same  size  of  grains,  and 
in  the  case  of  the  crushed  glass,  whose  grains  are  more  angular  than  those 
of  the  crushed  limestone,  which  have  a  tendency  to  be  cuboidal  in  form, 
the  pore  space  is  the  largest  of  all."8 

Seelheim  found  that  clays  when  allowed  to  settle  in  water  have  a  pore 
space  of  50  to  79  per  cent,  and  that  there  is  no  sensible  reduction  of  this 
space  under  a  pressure  of  30  meters  of  water/ 

In  clay  loams  and  clays  pore  spaces  as  high  as  48  to  52  per  cent  were 
obtained  by  King."  He  suggests  that  the  high  pore  space  of  clays  may 
possibly  be  partly  explained  by  the  angularity  of  the  grains,  it  being  well 
known  that  the  very  fine  mechanical  sediments  are  largely  composed  of 
angular  particles.* 

It.  is  evMent  from  these  experimental  results  of  King's  that  the  grains 
of  sands  and  soil  are  not  packed  by  nature  in  the  most  compact  manner 
possible;  otherwise  the  pore  spaces  would  run  lower,  rather  than  higher, 
than  Slichter's  minimum  pore  space  (25.95  per  cent);  for  the  natural  grains 

oSlichter,  cit,  p.  308. 

*Slichter,  cit.,  p.  309. 

«In  order  to  get  the  closest  packing,  the  material  was  added  "  in  small  lots  at  a  time  and  gently 
tamped  with  a  broad,  flatrfaced  pestle  until  the  vessel  was  filled.  .  .  .  The  vessel,  after  being  filled 
by  tamping,  was  '  struck  off '  with  a  piece  of  plate  glass,  then  held  firmly  while  with  ligh't  blows  the 
walls  of  the  tubes  were  struck  gently,  but  repeatedly,  as  long  as  any  reduction  in  volume  could  be 
produced."— King,  F.  H.,  Principles  and  conditions  of  the  movements  of  ground  water:  Nineteenth 
Ann.  Kept.  U.  S.  Geol.  Survey,  pt.  2,  1899,  p.  208. 

''King,  cit.,  p.  211. 

''King,  cit.,  p.  215. 

/Seelheim,  Zeitschr.  fiir  anal.  Chemie,  vol.  19,  p.  387;  cited  in  King,  F.  H.,  Principles  and  con- 
ditions of  the  movements  of  ground  water:  Nineteenth  Ann.  JJept.  U.  S.  Geol.  Survey,  pt.  2,  1899,  p.  78. 

?King,  cit,  pp.  213-215. 

A  King,  cit.,  pp.  217-218. 


PORE  SPACE  IN  ROCKS.  127 

of  soil  and  sand  are  not  spherical  in  shape,  or  of  uniform  size.  In  so  far  as 
the  grains  vary  from  regular  forms  and  uniform  magnitude  the  pore  space 
would  be  less  than  calculated;  but  in  so  far  as  the  method  of  packing  is 
not  the  most  compact  possible  the  pore  space  would  be  greater  than  calcu- 
lated. Thus  these  two  factors  neutralize  each  other  to  a  considerable 
degree,  and  we  are  obliged  to  turn  to  experiment  to  ascertain  approxi- 
mately the  facts.  It  is  probable  that  King's  experimental  results"  on  sands 
composed  of  well-rounded  grains  of  nearly  uniform  diameters,  where  the 
pore  space  was  between  32  and  40  per  cent,  represent  approximately  the 
original  pore  space  in  the  coarser  assorted  mechanical  sediments.  The 
more  porous  sandstones,  where  the  pore  space,  as  ascertained  by  Buckley, b 
varies  from  18  to  28  per  cent,  have  a  crushing  strength  varying  from  172 
to  413  kilograms  per  square  centimeter;  indeed,  are  strong  enough  to  serve 
for  building  stones.  It  is  clear  that  a  considerable  amount  of  cementing 
material  has  been  added,  and  that  the  pore  space  measured  is  much  less 
than  the  original  space  in  the  sands  before  cementation.  Hence  it  appears, 
both  from  experimental  work  by  King  and  by  deductions  from  actual 
measurements  of  the  space  in  partially  cemented  sandstones,  that  the 
original  pore  space  in  clean,  well-assorted  sands  probably  varies  from  one- 
fifth  to  as  much  as  two-fifths,  with  a  probable  average  of  about  one-third. 

It  is  much  more  difficult  to  give  a  statement  as  to  the  average  pore 
space  of  the  lavas.  Some  of  these  rocks  are  rather  dense  and  had  orig- 
inally a  very  small  amount  of  pore  space ;  others  are  exceedingly  vesicular 
and  originally  had  pore  spaces  amounting  to  50  to  75  per  cent,  or  even 
more.  It  is  rather  probable  that  where  a  succession  of  thin-bedded  basic 
lavas  are  piled  up  one  on  the  other,  as  in  the  Keweenawau  of  the  Lake 
Superior  region,  the  pore  space  averages  as  much  as  in  ordinary  sandstones; 
but  from  this  maximum  the  average  runs  down  as  the  lava  flows  become 
thicker  and  as  they  become  more  acid.  Therefore  the  average  pore  space 
of  the  vesicular  lavas  is  probably  not  more  than  one-third  to  one-half  as 
great  as  in  the  mechanical  sediments. 

It  is  even  more  difficult  to  make  an  estimate  of  the  amount  of  pore 
space  due  to  fractures  in  the  rocks,  such  as  faults,  joints,  fissility,  the  open- 

«King,  cit,  pp.  147-157. 

6  Buckley,  E.  R.,  Building  and  ornamental  stones  of  Wisconsin:  Bull.  Wisconsin  Geol.  and  Nat. 
Hist.  Surv.  No.  4,  1898,  pp.  393-395,  402-403. 


128  A  TREATISE  ON  METAMOHPHISM. 

ings  of  autoclastic  rocks,  etc.  In  the  case  of  some  breccias  the  pore  space 
is  certainly  as  large  as  in  the  mechanical  sediments,  and  such  breccias  in 
some  places  are  present  in  considerable  volume.  From  this  maximum 
amount  the  pore  space  of  course  varies  to  a  fraction  of  1  per  cent. 

I  am  therefore  wholly  unable  to  give  any  general  averages  of  the 
amount  of  pore  space,  taking  the  world  as  a  whole.  But  Shaler  has  esti- 
mated that  the  amount  of  igneous  and  vein  material  of  certain  regions  of 
the  New  England  coast  is  from  3  to  5  per  cent  of  the  superficial  area." 
Since  the  volumes  are  as  the  cubes  of  the  dimensions,  if  the  amount  of  vein 
material  were  the  same  in  other  directions  this  would  involve  a  filled  pore 
space  of  from  0.52  to  1.12  per  cent. 

From  the  foregoing  it  is  plain  that,  while  it  is  easy  to  ascertain  the 
amount  of  pore  space  in  a  given  rock,  it  is  very  difficult  indeed  to  make  any 
estimate  of  the  average  amount  of  pore  space  in  the  zones  of  katamorphism 
and  anamorphism.  It  is  shown  on  pages  187—191  that  these  zones  corre- 
spond, respectively,  to  the  zones  of  fracture  and  flowage.  It  is  certain  that 
the  pore  space  in  the  zone  of  fracture  is  far  greater  than  in  the  zone  of  flow- 
age.  It  is  also  equally  certain  that  the  pore  space  in  the  belt  of  weather- 
ing is  vastly  greater  than  in  the  belt  of  cementation.  When  these  various 
zones  and  belts  are  discussed  it  will  be  shown  that  both  the  unconsolidated 
materials  and  the  coherent  rocks  of  the  belt  of  weathering  are  exceedingly 
open  and  have  a  very  large  pore  space.  It  will  further  be  seen  that  in 
passing  downward  from  the  belt  of  weathering  to  the  belt  of  cementation 
there  is  a  sudden  diminution  in  the  amount  of  pore  space  available,  the 
rocks  becoming  almost  at  once  far  less  open.  Doubtless  on  the  average 
the  amount  of  pore  space  in  the  belt  of  cementation  steadily  diminishes 
from  the  upper  to  the  lower  part;  and  in  the  zone  of  anamorphism  the  pore 
space  is  almost  certainly  but  a  fraction  of  1  per  cent. 

It  is  to  be  remembered  that  below  the  comparatively  thin  belt  of 
weathering,  the  rocks,  with  unimportant  exceptions,  are  saturated.  Dana 
estimates  the  average  amount  of  water  contained  in  the  rocks  as  2.67  per 
cent  of  their  weight.6  Supposing  that  the  specific  gravity  of  the  crust  is  2.7, 
this  would  mean  a  pore  space  of  6.89  per  cent  of  the  volume  of  the  rocks; 
or,  if  the  rocks  were  saturated,  about  69  liters  of  water  in  every  cubic 

a  Shaler,  N.  8.,  The  crenitic  hypothesis  and  mountain  building:  Science,  vol.  11,  1888,  p.  281. 
*  Dana,  J.  D.,  Manual  of  geology,  American  Book  Co.,  4th  ed.,  1895,  pp.  205,  311. 


CHARACTER  OF  OPENINGS  IN  ROCKS.  129 

meter.  Supposing  the  pore  space  for  the  upper  part  of  the  zone  of  fracture 
to  be  one-fifth  of  that  suggested  by  Dana  and  to  diminish  to  zero  at  the 
lower  part  of  that  zone,  this  would  give  an  average  pore  space  for  that 
zone  of  0.69  per  cent.  Supposing  that  the  zone  of  fracture  extends  to  a 
depth  of  10,000  meters  and  that  the  pore  space  is  saturated,  the  amount  of 
contained  water,  if  concentrated  to  the  exclusion  of  rock,  would  make  a 
sheet  69  meters  thick,  extending  throughout  the  continental  areas.  This 
calculation  is  of  course  made  upon  an  hypothetical  basis  (see  pp.  569-571), 
but  it  shows  that  the  underground  water  is  truly  a  great  subterranean  sheet. 
This  subterranean  sheet  may  be  compared  to  the  blood  of  an  organism, 
and  the  comparison  has  force  to  the  degree  that  it  is  the  chief  medium 
through  which  the  transformations  of  the  rocks  are  accomplished. 

CIRCULATION    OF   GROUND   WATER. 

Subterranean  water  must  be  considered  from  two  points  of  view- 
its  circulation  and  its  work. 

The  actual  ground-water  circulation  depends  upon  the  openings  in  the 
rocks,  the  forces  producing  water  circulation,  and  the  forces  opposed  to 
circulation. 

OPENINGS  IN  ROCKS. 

The  rate  and  amount  of  flowage  of  water  is  largely  dependent  upon 
the  openings  in  rocks.  The  openings  in  rocks  in  reference  to  flowage 
need  to  be  considered  from  the  following  points  of  view:  The  form  and 
continuity  of  the  openings,  the  size  of  the  openings,  and  the  percentage  of 
openings,  or  pore  space. 


FORM    AND   CONTINUITY    OF   OPENINGS. 


For  a  given  cross  section,  in  proportion  as  an  opening  approaches  a 
circular  form — that  is,  as  it  approaches  a  minimum  of  wall  area  per  unit  of 
volume— the  flow  increases,  because  the  friction  between  the  moving  water 
and  the  film  of  fixed  water  upon  the  walls  is  less  per  unit  volume.  In 
proportion  as  the  openings  are  continuous  in  rocks  the  flow  increases. 

The  openings  in  rocks  include  (1)  those  which  are  of  great  length  and 
breadth  as  compared  with  their  width,  and  thus  are  essentially  flat  parallel- 
epipeds;  (2)  those  in  which  the  dimensions  of  the  cross  sections  of  the 
openings  are  approximately  the  same,  and  therefore  resemble  tubes  of 
various  kinds;  and  (3)  irregular  openings. 

5ION   XLVII — 04 9 


130  A  TREATISE  ON  METAMORPHISM. 

(1)  The  openings  which  have  great  length  and  breadth  as  compared 
with  their  width  are  those  of  bedding  partings,  of  faults,  of  joints,  and  of 
fissility.  It  is  recognized  that  many  of  the  fractures  are  exceedingly 
complex.  They  are,  indeed,  in  many  instances  a  series  of  parallel  or 
intersecting  fractures,  forming  a  zone  of  brecciatiou.  However,  for  such  a 
a  zone,  as  a  whole,  the  statement  still  holds  that  the  openings  have  great 
length  and  depth  as  compared  with  their  width. 

Bedding  partings  are  parallel  to  the  layers.  Since  ground  waters 
very  frequently  follow  formations,  the  bedding  partings  become  important 
factors  in  the  promotion  of  flowage  parallel  to  the  formation  This  is 
especially  true  of  the  contacts  of  formations  of  different  character.  These 
contacts  are  places  of  maximum  differential  movements,  of  consequent  com- 
plex fracturing,  and  therefore  of  important  openings  and  large  circulation. 

In  position  the  fault,  joint,  and  fissile  openings  ordinarily  have  an 
important  vertical  element,  or  at  least  traverse  the  beds.  Frequently  they 
are  nearly  vertical,  or  traverse  layers  or  formations  at  right  angles.  In 
consequence  of  this  they  are  very  important  factors  in  the  vertical  move- 
ments of  ground  water. 

As  to  continuity,  bedding  partings  are  likely  to  be  the  most  continuous ; 
faults  come  next  in  continuity,  joints  next,  and  fissile  openings  are  those 
that  are  least  continuous. 

Bedding  partings  are  likely  to  be  continuous  for  long  distances,  and 
because  of  this  and  their  size  (considered  on  pp.  137-138),  they  are  fre- 
quently important  factors  in  the  flowage  of  ground  water." 

Faults  may  have  very  great  continuity.  Thrust  faults  of  15  kilometers 
and  more  along  the  dip  are  known;  and  along  the  strike  faults  may  extend 
for  even  hundreds  of  kilometers,  although  ordinarily  their  extent  is  much 
less.  From  their  great  persistence  and  from  the  fact  that  they  are  likely  to 
cut  across  formations,  thus  frequently  severing  and  displacing  impervious 
strata  and  consequently  connecting  porous  strata  separated  by  impervious 
strata  with  one  another,  faults  are  of  very  great  consequence  in  the  flowage 
of  ground  water. 

Joints  are  less  extensive  than  faults,  but  they  may  extend  across  an 
entire  formation,  or  even  across  two  or  more  contiguous  formations.  The 

"King,  F.  H.,  Principles  and  conditions  of  the  movements  of  ground  water:     Nineteenth  Ann. 
Kept.  U.  S.  Geol.  Survey,  pt.  2,  1899,  p.  126. 


NATURE  OF  OPENINGS.  131 

extent  of  joints  along-  the  strike  may  be  many  kilometers.  While  joints 
are  less  extensive  than  faults,  they  are  far  more  numerous.  Probably  their 
number,  as  compared  with  faults,  more  than  compensates  for  their  lack  of 
extent.  Joints  are  therefore  of  very  great  importance  in  the  flowage  of 
ground  water.  On  the  average  the}'  may  be  of  even  greater  importance 
than  faults.  Joints,  like  faults,  may  connect  separated  porous  strata,  but 
very  frequently  the  joints  do  not  pass  through  the  relatively  plastic 
separating-  impervious  strata,  and  therefore  in  this  respect  are  of  less 
consequence  than  faults. 

Fissility  openings  usually  have  less  extent  than  bedding  partings, 
faults,  or  joints;  and  the  openings  are  small.  While  they  doubtless  have  an 
important  influence  in  water  flowage.  they  are  not  of  such  consequence  as 
bedding  partings,  faults,  or  joints. 

(2)  Openings  in  which  the  dimensions  of  the  cross  sections  are 
approximately  the  same  are  those  of  the  mechanical  deposits,  including 
conglomerates,  sandstones,  soils,  tuffs,  etc. 

The  openings  of  mechanical  sediments  have  a  strong  tendency  to  a 
definite  form,  and  are  continuous.  The  forms  of  these  openings  have  been 
fully  discussed  by  Slichter.0  The  openings  alternately  narrow  and  widen. 
At  the  wider  parts  their  sections  are  roughly  polygonal,  the  polygons 
having  more  than  three  sides,  and  these  curved.  At  their  narrowest  places 
the  cross  sections  of  the  openings  approximate  triangles,  and  where  the 
grains  are  of  equal  size  the  triangles  are  equilateral.  The  form  of  the 
tubes  at  their  minimum  is  due  to  the  contact  of  three  grains  in  a  plane, 
the  space  between  which  is  nearly  triangular.  (Fig.  3.) 

Professor  Slichter  has  further  shown  that  there  are  various  possible 
natural  systems  of  packing  of  particles.  In  nature  one  system  of  packing 
may  hold  for  a  certain  distance,  and  then  be  replaced  by  another  system. 
Within  any  system  of  packing  all  the  openings  are  connected  with  one 
another  by  straight  or  curved  tubes,  triangular  at  their  minimum  cross 
section,  and  no  opening  is  shut  off  from  any  other  opening.  Slichter  has 
shown  that  in  the  various  natural  systems  of  packing  of  the  particles  there 
is  at  least  one  direction  in  which  the  tubes  are  straight;  in  other  words, 
there  is  one  direction  in  which  a  straight  wire  may  be  thrust  without  coming 

°  Slichter,  C.  S.,  Theoretical  investigation  of  the  motion  of  ground  water:  Nineteenth  Ann.  Kept. 
U.  S.  Geol.  Survey,  pt.  2,  1899,  pp.  305-323. 


132  A  TREATISE  ON  METAMORPHISM. 

in  contact  with  any  grain.  (Fig.  4.)  In  any  other  than  the  one  direction, 
where  the  grains  are  naturally  arranged,  the  tubes  are  ordinarily  interrupted 
In  any  case  the  continuity  of  the  tubes  in  straight  lines  persists  so  far  as  the 
arrangement  of  grains  is  by  one  system  of  piling.  Slichter  has  shown  that 
the  openings  in  the  directions  in  which  the  tubes  are  not  straight  may  be 
neglected  so  far  as  the  flowage  of  water  is  concerned;  he  therefore  con- 
cludes the  quantity  of  flowage  to  be  dependent  upon  the  continuous  straight 
or  nearly  straight  tubes.  These  of  course  vary  in  size,  but  the  water 
may  be  reckoned  as  passing  through  continuous  tubes  of  the  minimum  size, 
made  by  the  cross  section  between  three  grains  arranged  in  a  plane  at  right 
angles  to  the  direction  of  the  tubes.  Of  course  it  is  understood  that  any 


FIG.  3.— Triangular  cross  sections  of  pore  space.    After  Slichter, 

one  system  of  arrangement  does  not  extend  indefinitely,  and  that  where 
one  system  of  packing  changes  into  another  there  are,  ordinarily,  bends  in 
the  tubes. 

Slichter  further  shows  that  the  amount  of  space  in  mechanical  sediments 
before  cementation  takes  place  is  largely  dependent  upon  the  system  of 
packing.  It  is  also  dependent  upon  the  regularity  of,  the  grains  and  their 
variation  in  size.  The  more  nearly  spherical  the  grains  and  the  more 
nearly  uniform  the  size,  the  greater  is  the  pore  space. 

Ordinarily  the  continuous  tubes  of  mechanical  sediments  are  limited 
by  the  boundaries  of  a  stratum  or  formation.  However,  a  porous  formation 
may  extend  for  hundreds  of  kilometers  and  have  a  thickness  of  hundreds 


IMPORTANCE  OF  OPENINGS  IN  SANDSTONES. 


133 


of  meters.  This  is  well  illustrated  by  the  strata  bearing  artesian  water, 
many  of  which  certainly  transmit  great  quantities  of  water  for  hundreds  of 
kilometers.  An  excellent  illustration  of  porous  strata  of  this  class  is  the 
Dakota  sandstone.  This  sandstone  yields  great  quantities  of  water  along 
the  James  River  Y alley,  and  the  nearest  feeding  area,  so  far  as  known,  is 
in  the  Black  Hills,  400  kilometers  distant.  The  volume  of  water  which  issues 


FIG.  4. — Spheres  packed  in  the  most  compact  manner  possible,  showing  continuous  open- 
ing* in  one  direction.    After  Sliehter. 

from  sandstone  strata  in  artesian  basins  shows  how  important  is  the  class  of 
openings  tinder  consideration. 

Since  the  continuous  openings  of  sediments  are  commonly  limited  to 
a  formation,  it  is  plain  that  such  openings  are  very  favorable  to  the  flowage 
of  water  along  a  formation,  but  are  less  potent  in  the  transference  of  water 
from  one  stratum  or  formation  to  another. 

(3)  Irregular  openings   are  those  of  the  vesicular  lavas  and  of   the 


134  A  TREATISE  ON  METAMORPHISM. 

irregular  fractures  of  rocks.  In  rocks  where  the  openings  are  exceedingly 
irregular  in  form  the  flowage  of  water  is  limited  by  the  continuous  openings, 
however  small  they  may  be. 

Irregular  openings  may  be  of  any  form.  In  the  lavas  they  are  fre- 
quently spherical  or  ovoid.  In  the  compact  rocks  they  are  confined  to  the 
very  minute,  exceedingly  irregular  interspaces  between  the  mineral  par- 
ticles, which  apparently  are  in  perfect  contact.  As  already  seen,  in  the  very 
vesicular  lavas  the  pore  space  may  vary  from  a  small  per  cent  to  a  very 
large  amount,  even  to  75  per  cent  or  more.  The  openings  are  more  likely 
to  be  continuous  where  the  pore  space  is  large  than  where  it  is  small.  But 
even  where  the  pore  space  is  very  large  the  openings  of  lavas  are  not  nearly 
so  continuous  nor  the  minima  of  the  tubes  so  large  as  in  sands.  In  the 
igneous  rocks  and  in  the  rocks  metamorphosed  under  deep-seated  conditions 
the  openings  are  minute;  they  are  controlled  by  the  form  of  the  grains. 
They  are,  therefore,  very  irregular  and  discontinuous. 


SIZE    OK    ol'KXIXfiS. 


The  size  of  the  openings  is  very  important  in  the  circulation  of  ground 
water.  The  size  of  openings  must  be  discriminated  from  the  amount  of 
pore  space.  The  amount  of  pore  space  may  be  the  same  in  two  cases,  but 
in  one  the  openings  may  be  very  few  and  large,  and  in  the  other  very 
numerous  and  small.  The  flowage  in  the  two  cases,  other  conditions  being 
equal,  is  very  different,  For  a  given  mass  of  water  the  internal  friction, 
both  within  the  moving  water  and  between  the  moving  and  fixed  water 
increases  very  greatly  as  the  openings  decrease  in  size.  It  is,  therefore, 
necessary  to  consider  the  various  classes  of  openings  in  reference  to  size. 

Upon  the  basis  of  size  openings  in  rocks  may  be  divided  into  (a)  open- 
ings larger  than  those  of  capillary  size,  or  supercapillary  openings;  (b) 
capillary  openings,  and  (c)  openings  smaller  than  those  of  capillary  size,  or 
subcapillary  openings. 

For  water,  openings  larger  than  capillary  openings,  according  to 
Daniell,"  may  be  circular  tubes  which  exceed  0.508  mm.  in  diameter,  or  may 
be  sheet  openings,  such  as  bedding  partings,  faults,  joints,  etc.,  the  widths  of 
which  exceed  one-half  of  this,  or  0.254  mm.  To  movement  of  water  in  such 


"Paniell,  Alfred,  A  text-book  of  the  principles  of  physics,  3d  ed.,  Macmillan  Co.,  New  York, 
1895,  pp.  315-317. 


SIZE  OF  OPENINGS  IN  ROCKS.  135 

openings  the  ordinary  laws  of  hydrostatics  apply.  Capillary  openings  for 
water  solutions  include  those  which,  if  circular  tubes,  are  smaller  than 
0.508  mm.  in  diameter,  and  those  which,  if  sheet  spaces,  are  narrower  than 
0.254  mm.,  and  which  in  either  case  are  larger  than  the  openings  in  which 
the  molecular  attractions  of  the  solid  material  extend  across  the  space. 
Such  openings  in  the  case  of  circular  tubes  are  those  smaller  than  0.0002  mm. 
in  diameter,  or,  if  sheet  passages,  are  below  0.0001  mm.  in  width.  Capil- 
lary openings,  therefore,  include  circular  tubes  from  0.508  to  0.0002  mm. 
in  diameter,  and  sheet  passages  from  0.254  to  0.0001  mm.  in  width.  Capil- 
lary openings  of  other  forms  have  a  range  limited  between  0.508  and 
0.0001  mm.,  but  no  one  form  has  so  wide  a  range  as  this.  To  movement 
of  water  in  openings  such  as  these  the  laws  of  capillary  flow  apply.  By 
subcapillary  openings  are  meant  those  in  which  the  attraction  of  the  solid 
molecules  extends  from  wall  to  wall.  These  include  all  tubes  smaller  than. 
0.0002  mm.  in  diameter,  and  sheet  openings  smaller  than  0.0001  mm.  in 
width.  For  intermediate  forms  the  subcapillary  openings  have  as  their 
maximum  limit  a  range  from  0.0002  to  0.0001  mm. 

It  is  not  supposed  that  supercapillary  openings,  capillary  openings, 
and  subcapillary  openings  are  sharply  separated  from  one  another.  They 
grade  into  one  another,  and  the  laws  below  given  which  control  the  flowage 
in  one  class  of  openings  are  gradually  modified  until  they  pass  into  the 
laws  which  control  the  flowage  in  another  class  of  openings.  For  instance, 
water  in  circular  tubes  slightly  larger  than  0.508  mm.  in  diameter  would 
to  some  extent  obey  the  laws  of  flowage  of  capillary  openings,  and  water 
in  tubes  slightly  less  than  0.508  mm.  in  diameter  would  to  some  extent 
obey  the  laws  of  supercapillary  flow.  In  short,  flowage  in  openings  near 
the  dividing  line  between  two  classes  obeys  laws  intermediate  between 
those  controlling  flowage  in  the  typical  cases  of  each  class. 

The  areas  of  openings  of  variable  size  and  similar  form  vary  as  the 
squares  of  their  respective  diameters.  The  circumferences  of  openings  of 
variable  size  and  similar  form  vary  as  their  respective  diameters.  It  follows, 
for  a  given  volume  of  water,  that  the  larger  the  openings  in  which  it  is 
contained  the  less  is  the  surface  of  contact.  For  instance,  if  for  an  opening 
of  any  form,  of  given  diameter,  the  surface  of  contact  for  1  cm.  of  length 
be  1  sq.  cm.,  if  the  cross  diameter  be  doubled,  the  length  remaining  the 
same,  the  volume  of  the  water  is  four  times  as  great,  but  the  surface  of 


136  A  TREATISE  ON  METAMORPHISM. 

contact  is  only  twice  as  great.  If  the  diameters  be  decreased  to  one-third, 
the  volume  of  the  water  is  decreased  to  one-ninth,  but  the  surface  of  contact 
to  one-third  only. 

As  a  consequence  of  the  relation  between  size  of  openings  and  area  of 
contact,  it  follows  that  in  small  openings  a  given  volume  of  water  is  capable 
of  performing  much  more  work  upon  the  rocks  than  in  openings  of  larger 
size,  for  the  surfaces  of  contact  are  the  places  where  chemical  interaction 
between  the  water  and  rock  takes  place.  How  important  is  the  factor  of 
small  size  in  the  amount  of  work  which  may  be  accomplished  by  ground 
water  can  be  adequately  comprehended  only  when  the  surface  of  action  for 
a  given  volume  of  water  for  small  openings  is  calculated.  To  illustrate,  if 
the  openings  are  circular  tubes  of  a  size  at  the  border  line  between  those  of 
Bupercapillary  and  capillary  size — that  is,  tubes  0.508  mm.  in  diameter — 1 
cu.  cm.  of  water  would  have  a  surface  contact  with  the  rocks  of  about  -78.74 
sq  cm.  If  the  openings  be  sheet  openings  at  the  boundary  between  super- 
capillary  and  capillary — that  is,  0.254  mm.  in  width — 1  cu.  cm.  of  water 
would  have  a  surface  contact  of  about  78.74  sq.  cm.  If  the  openings  be 
circular  tubes  at  the  border  line  between  those  of  capillary  and  subcapillarv 
openings— that  is,  0.0002  mm.  in  diameter — 1  cu.  cm.  of  water  would  have 
a  surface  contact  of  about  200,000  sq.  cm.  If  the  openings  be  sheet  open- 
ings at  the  border  line  between  those  of  capillary  and  subcapillary  size- 
that  is,  have  a  width  of  0.0001  mm. — 1  cu.  cm.  of  water  would  have  a 
surface  contact  of  200,000  sq  cm.  Therefore  1  cu.  cm.,  or  1  gram  of  water, 
has  a  surface  contact  varying  from  0.007874  to  20  square  meters  in  circular 
capillary  tubes;  and  in  sheet  passages  has  a  surface  contact  varying  from 
O.OU7874  to  20  square  meters.  It  has  been  calculated  by  Whitney  that 
"the  grains  in  a  cubic  foot  of  soil  have,  on  the  average,  no  less  than  50,000 
square  feet  of  surface  area."0  The  magnitude  of  these  numbers  shows  how 
important  a  factor  in  the  work  of  a  given  volume  of  ground  water  is  the 
size  of  the  openings  in  which  the  water  is  contained. 

It  follows  from  the  above  relations  that  the  area  of  contact,  and 
therefore  the  friction  between  moving  water  and  the  fixed  film  of  water 
adherent  to  the  walls,  is  inversely  as  the  size  of  the  openings.  As  will  be 

"  Whitney,  Milton,  The  physical  principles  of  soils  in  their  relations  to  moisture  and  crop  distri- 
bution: Bull.  Weather  Bureau  No.  4,  U.  S.  Dept.  of  Agric.,  1892,  p.  14. 


FLO  WAGE  IN  SUPERCAP1LLARY  OPENINGS.  137 

seen,  this  is  a  matter  of  controlling  consequence  in  flowage  in  small  and 
especially  in  very  small  openings. 

supercapuiary  openings — rpiie  flowage  of  water  through  supercapillaiy  tubes 
is  controlled  by  the  ordinary  laws  of  hydrokinetics.  Ignoring  friction, 
the  flowage  of  water  is  as  the  square  root  of  the  pressure  or  head.  If 
Vrrvelocity,  H=head,  and  G=force  of  gravity,  then  V  per  second 
=  \/2GH.  For  instance,  the  velocity  resulting  from  a  pressure  of  1  atmos- 
phere or  a  head  of  1033.3  cm.  would  be  the  square  root  of  2  X  981  X 
1033.3  —  1423.8  cm.  per  second." 

This  formula  is  only  approximately  correct,  for  the  internal  friction  in 
supercapillary  tubes  is  dependent  upon  the  viscosity  of  the  solutions,  upon 
the  regularity  of  the  openings,  upon  their  length  and  size,  and  upon  the 
velocity  of  flowage.  If  the  openings  be  not  straight,  or  vary  in  size,  or 
both,  eddies  form,  which  increase  the  internal  friction  and  decrease  the 
speed  of  movement.  The  friction  between  the  moving  liquid  and  that  fixed 
to  the  walls  increases  with  increase  of  length,  with  decrease  of  size,  with 
roughness  of  surface,  and  with  increase  in  velocity.  If  the  available  area 
be  great  and  the  movement  consequently  very  slow,  the  resistance  per  unit 
of  length  due  to  friction  becomes  so  small  as  to  be  almost  inappreciable. 
But  even  if  the  openings  be  large  and  continuous  the  formula  gives  some- 
what too  high  results.  If  the  flow  be  rapid  in  long,  rough,  irregular 
underground  passages,  the  resistance  is  so  great  as  to  make  the  formula 
above  given  inapplicable. 

Supercapillary  openings  include  the  greater  number  of  bedding  part- 
ings, fault  openings,  joint  openings,  some  of  the  openings  of  fissility,  and 
the  openings  in  the  coarser  mechanical  sediments,  such  as  coarse  sandstones 
and  conglomerates.  The  distance  from  an  angle  to  the  opposite  side  of  the 
roughly  triangular  tubes  (fig.  3,  p.  132)  in  sandstones  composed  of  spherical 
grains  of  equal  size,  which  average  3  nun.  in  diameter,  somewhat  exceeds 
0.508  mm.*  The  average  diameter  of  the  pores  in  the  system  of  closest 
packing  is  43  per  cent  greater  than  the  minimum  section  of  the  triangular 
pores.5  It  therefore  follows  that  a  sediment  composed  of  grains  just  large 

f'Daniell,  Alfred,  A  text-book  of  the  principles  of  physics,  3d  ed.,  Macmillan  Co.,  New  York,  1895, 
p.  303. 

fcSlichter,  C.  8.,  Theoretical  investigation  of  the  motion  of  ground  water:  Nineteenth  Ann. 
Kept.  U.  S.  Geol.  Survey,  pt.  2,  1899,  p.  316. 

<•  Slighter,  cit.,  p.  317. 


138  A  TREATISE  ON  METAMOKPHISM. 

enough  to  make  the  pores  capillary  at  the  smallest  section  have  super- 
capillarv  p<  »res  in  other  parts  of  the  section.  Hence,  it  may  be  said  that 
sandstones  and  conglomerates  the  grains  of  which  exceed  3  mm.  in  diameter 
have  tubes  which  are  greater  than  those  of  capillary  size.  But  the  grains 
in  the  great  majority  of  sandstones  average  less  than  3  mm.  in  diameter, 
and  hence  the  pore  openings  in  sandstones  are  for  the  most  part  capillary, 
and  are  considered  under  the  next  heading. 

It  is  through  openings  exceeding  those  of  capillary  size — that  is,  cir- 
cular tubes  larger  than  0.508  mm.  in  diameter  and  sheet  openings  greater 
than  0.254  mm.  in  diameter — that  the  rapid  circulation  of  underground 
water  is  accomplished.  For  instance,  the  openings  through  which  springs 
of  large  size  issue  mainly  exceed  those  of  capillary  dimensions. 

capniary  openings. — Capillary  openings  include  the  great  majority  of  the 
openings  of  sands  and  sandstones,  many  of  the  openings  of  fine  conglom- 
erates, many  of  the  openings  of  porous  lavas,  and  many  of  the  openings 
produced  by  fracture.  As  already  noted,  the  superior  limit  of  size  of 
grains  of  sands  and  sandstones  composed  of  grains  of  uniform  size,  the 
smallest  openings  of  which  are  capillary,  is  3  mm.  in  diameter.  The  inferior 
limit  of  size  are  grains,  the  diameters  of  which  are  six  times  the  maximum 
diameter  of  subcapillary  tubes,  or  0.0012  mm.  The  majority  of  the  par- 
ticles of  most  clays,  shales,  and  slates  are  much  smaller  than  this,  and 
therefore  the  openings  of  these  rocks  are  subcapillary.  Hence  capillary 
openings  in  mechanical  sediments  range  from  very  fine  sands  to  very 
coarse  sands.  Many  of  the  openings  of  fissility  are  capillary;  but  the 
majority  of  bedding  partings,  fault  openings,  and  joint  openings  are  partly 
supercapillary,  although  often  the  walls  of  such  fractures  are  so  close 
together  as  to  make  even  these  openings  capillary  in  part. 

In  capillary  openings  the  resistance  to  flow  increases  very  rapidly  as  a 
tube  diminishes  in  size.  This  is  due  to  the  fact,  already  explained,  that 
the  area  of  contact  between  the  moving  liquid  and  that  fixed  to  the  wall 
increases  inversely  as  the  size  of  the  openings.  Indeed,  the  friction  between 
the  moving  and  the  fixed  liquid  becomes  the  dominant  factor  in  the  resist- 
ance to  flowage  in  capillary  tubes.  As  openings  decrease  in  size,  at  the 
diameter  at  which  this  factor  controls  for  a  given  liquid  the  openings 
become  of  capillary  size  for  that  liquid. 


FLO  WAGE  IN  CAPILLARY  OPENINGS.  139 

According  to  Poiseuille,  the  general  formula  for  the  flow  through  a 
tube  of  circular  section  is 

J~ 

in  which  /is  the  discharge  in  cubic  centimeters  per  second,  a  is  the  radius 
of  the  tube,  I  its  length,  p  is  the  difference  in  pressure  at  its  ends  in  dynes 
per  square  centimeter,  and  fi  is  the  coefficient  of  viscosity  of  the  liquid.01 
According  to  Slichter,  "if  A  is  the  area  of  cross  section,  this  formula  may 
be  written 

f-  A'P 

•>-  Hx /if 

and  the  mean  velocity  of  the  fluid  in  the  tube  is  given  by 

^^  =  (0-03979)^"" 
In  a  triangular  tube  the  flow  per  second  is  represented  by  the  formula 


and  the  velocity  by  the  formula 

v= (0.02887)^ 

t**          . 

"The  mean  velocity  for  a  circular  tube  of  equivalent  area  of  cross  section 
was  found  to  be  about  38  per  cent  more."6  Slichter  finds  the  volume  and 
velocity  of  flow  in  an  elliptical  cylinder  to  vary  but  slightly  from  that  of 
a  circular  tube.  "Even  an  eccentricity  of  0.866  will  change  the  flow  by 
but  10  per  cent,  and  an  eccentricity  of  one-half  will  reduce  the  flow  by 
about  one-half  of  1  per  cent.  Thus  it  is  clear  that  a  slight  change  in  the 
shape  of  the  cross  section  of  a  tube  will  change  but  slightly  the  flow 
through  it.  Analogy  wan-ants  us  in  extending  this  truth  to  tubes  having 
other  than  elliptical  sections.  For  example,  we  may  conclude  that  the  flow 
through  a  tube  whose  section  is  an  oblique  triangle  is  given  approximately 
by  the  formula  for  a  tube  whose  section  is  an  equilateral  triangle  of  the 
same  area,  even  though  the  shape  of  the  section  of  the  given  tube  differs 
slightly,  or  even  materially,  from  that  of  an  equilateral  triangle."4  Further- 

"  Slichter,  C.  S.,  Theoretical  investigation  of  the  motion  of  ground  water:  Nineteenth  Ann.  Kept. 
U.  S.  Geol.  Survey,  pt.  2,  1899,  p.  317. 
*  Slichter,  cit.,  p.  319. 


140  A  TREATISE  ON  METAMORPHISM. 

more,  in  capillary  tubes  "the  velocity  of  flow  through  a  tube  of  variable 
section  will  be  less  than  the  velocity  of  flow  through  a  tube  having  a 
uniform  section  equal  to  the  mean  section  of  the  first  tube,  because  of 
the  viscosity  or  internal  friction  of  the  expanding  or  contracting  stream."" 

Daniell  expresses  a  part  of  the  laws  of  capillary  flow  in  words,  instead 
of  in  a  formula,  as  follows:  "The  flow  in  capillary  tubes  is  proportional  not 
to  the  square,  but  to  the  fourth  power  of  the  radius;  the  velocity  is  propor- 
tional not  to  the  square  root  of  the  pressure,  but  to  the  pressure  itself. 
The  resistance  in  capillary  tubes  varies  directly  as  the  velocity;  in  wide 
tubes  approximately  as  the  square  of  the  velocity.  This  seems  discrepant; 
but  it  is  due  to  the  formation  of  eddies  in  the  wider  tubes;  in  a  capillary 
tube  the  flow  is  steady.'"1 

From  the  foregoing  it  follows  that  the  flow  in  a  tube  with  a  radius 
one-fifth  millimeter  in  diameter  is  sixteen  times  as  great  as  in  a  .tube 
one-tenth  millimeter  in  diameter.  Furthermore,  in  a  tube  of  any  definite 
length,  if  the  pressure  be  doubled  the  flow  is  doubled;  if  trebled  the  flow  is 
trebled,  etc.  However,  experimental  work  by  King  upon  the  flowage  of 
water  through  capillary  openings  of  sandstones  and  sands  gave  results 
showing  that  under  the  conditions  in  which  he  performed  his  experiments 
the  flowage  increased  faster  than  the  pressure.  The  pressure  in  the  experi- 
ments varied  from  a  small  fraction  of  an  atmosphere  to  somewhat  more  than 
an  atmosphere.  The  departure  from  I'oiseuille's  law  varied  from  less  than 
1  per  cent  to  more  than  50  per  cent.'  In  the  experiments  the  departures 
seemed  to  be  greater,  on  the  average,  when  very  low  pressures  were  used 
than  when  moderate  pressures  were  used.  The  very  variable  results  nmv 
be  partly  explained  by  the  conditions  under  which  the  experiments  were 
performed,  but  it  is  entirely  possible  that  the  departures  are  partly  to  be 
explained  by  the  relative  importance  of  internal  friction  due  to  viscosity 
when  the  rates  of  movements  are  slow.  (See  pp.  141-143.) 

Also,  according  to  Poiseuille's  law,  the  flowage  is  inversely  as  the 
viscosity.  When  it  is  remembered  that  the  viscosity  of  water  decreases 
rapidly  with  increase  of  temperature,  it  is  seen  that  this  is  a  very  important 

"Slighter,  C.  S.,  Theoretical  investigation  of  the  motion  of  ground  water:  Nineteenth  Ann.  Kept. 
U.  S.  Geol.  Survey,  pt.  2, 1899,  p.  320. 

f> Daniell,  Alfred,  A  text-)x>ok  of  the  principles  of  physics,  3d  ed.,  Macmillan  Co.,  New  York, 
1895,  p.  316. 

'King,  F.  H.,  Principles  and  conditions  of  movements  of  ground  water:  Nineteenth  Ann.  Kept. 
U.  S.  Geol.  Survey,  pt.  2,  1899,  pp.  135-157. 


FLOWAGE  IN  CAPILLARY  OPENINGS.  141 

factor.     The  relative  viscosity   of  water  at  various    temperatures    below 
100°  C.  is  as  follows:" 

R<lti1i  r,   I'lKi'uxifij  of  water  at  rfffi-rent  temperatures. 


0°  .............................................................  100.00 

15°  ........................................  ......................  63.60 

30°  .............................................................  44.  90 

4o°  .............................................................  33.89 

60°  .........................................................  ....  26.  94 

75°  .............................................................  21.75 

90°  ......  .  ......................................................  18.16 

From  this  table  it  appears  that  the  viscosity  of  water  at  45°  C.  is  about 
one-third  its  viscosity  at  0°  C.;  at  90°  C.,  less  than  one-fifth  as  great  as 
at  0°  C.  It  therefore  follows  that  temperature  is  a  factor  of  the  greatest 
importance  in  the  flowage  of  water  through  capillary  openings  in  the  litho- 
sphere.  It  is  shown  (pp.  138,  145-146)  that  the  openings  in  the  lithosphere 
are  largely  those  of  capillary  or  subcapillary  size  ;  hence  the  importance  of 
the  temperature  element. 

Another  factor  entering  into  the  flowage  of  ground  water  is  the 
influence  of  the  meniscus  where  the  openings  are  not  fully  occupied  by 
water.  Wolff6  has  shown  that  if  water  be  introduced  into  an  empty 
capillary  tube,  the  meniscus  in  advance  of  the  column  is  an  important 
retarding  influence,  and  consequently  that  the  movement  is  slower  than 
under  circumstances  where  there  is  no  meniscus.  This  influence  is  likely 
to  be  important  in  many  cases  in  the  belt  of  weathering,  where  partial 
filling  is  the  rule,  but  is  probably  of  little  consequence  in  the  belt  of 
cementation  below  the  level  of  ground  water,  where  saturation  is  the  rule. 

In  conclusion,  it  should  be  fully  understood  that  the  laws  of  capillary 
flow,  as  developed  by  Poiseuille  and  others,  involve  rather  rapid  movement 
through  the  capillary  openings.  It  has  already  been  stated  that  viscosity 
of  the  solutions  and  friction  between  the  moving  and  the  fixed  water  are 
the  determinative  factors  in  reference  to  capillary  flow.  It  is  highly 
probable  that  where  the  movements  are  very  slow  the  friction  is  minute  or 
inappreciable  and  that  the  consequent  departures  from  Poiseuille's  laws  are 
very  great.  Apparently  in  the  exceedingly  slow  movements  of  many  of 

«Landolt  and  Bernstein  Tabellen,  1894,  p.  288;  supplemented  by  experimental  data  furnished  by 
Mr.  C.  F.  Bo  wen. 

6  Wolff,  H.  C.,  The  unsteady  motion  of  viscous  liquids:  Trans.  Wisconsin  Acad.  Sci.,  Arts,  and 
Letters,  vol.  12,  pt.  2,  1900,  pp.  552-553. 


142  A  TREATISE  ON  METAMORPH1SM. 

the  larger  masses  of  ground  water  the  viscosity  of  water  and  the  friction 
becomes  almost  zero  per  unit  area.  Evidence  of  this  is  furnished  by  the 
fact  that  artesian  water  flowing  through  rocks  for  hundreds  of  kilometers, 
the  openings  <>f  which  are  capillary,  may  have  nearly  the  full  pressure  due 
to  head.  For  instance,  the  artesian  water  adjacent  to  Lake  Michigan  at 
Chicago  at  the  early  wells,  before  they  became  so  numerous  as  to  interfere 
when  allowed  to  flow,  had  a  head  of  30  meters  above  the  surface,  and  the 
feeding  area  is  only  about  80  meters  above  Chicago;0  yet  the  water  has 
traveled  underground  from  150  to  250  kilometers.  The  resistance  causing 
the  loss  of  head  of  50  meters  is  to  be  distributed  through  this  distance; 
therefore  the  friction  per  meter  must  have  approached  an  infinitesimal 
amount.  The  same  thing  is  again  finely  illustrated  by  the  artesian  wells 
of  the  James  River  Valley  of  South  Dakota.  The  water  of  these  wells 
must  have  traveled  at  least  from  the  eastern  border  of  the  Black  Hills,  400 
kilometers.  The  elevation  at  the  source  is  1,500  meters  and  at  the  James 
River  500  meters.  The  consequent  loss  of  head  of  considerably  less  than 
1,000  meters  is  due  to  resistance  through  the  entire  distance,  and  again  must 
be  almost  immeasurably  small  per  meter.6  In  all  such  instances  the  average 
movement  is  exceedingly  slow,  for  it  will  be  shown  that  to  accomplish  the 
first  of  the  above  journeys  more  than  a  century  was  perhaps  required,  and 
for  the  second  possibly  centuries  were  necessary.  (See  pp.  585-586.) 

But  the  moment  the  speed  of  movement  becomes  appreciable  the  resist- 
ance promptly  runs  up.  This  is  shown  by  the  very  slow  fall  of  a  slanting 
water  table  in  sands  as  the  result  of  lateral  flowage.  The  best  illustration  of 
this  of  which  I  know  is  that  kindly  furnished  me  by  J.  B.  Lippincott,  city 
engineer,  of  Los  Angeles,  Gal.  The  Los  Angeles  River  is  mainly  fed  by 
ground  waters  derived  from  granitic  and  other  sands  which  are  of  moderate 
coarseness,  but  the  openings  of  which  are  capillary.  The  water  table 
rises  from  the  headwaters  of  the  river  to  a  point  north  of  Fernando — about 
16.1  kilometers — from  a  little  more  than  180  meters  to  a  little  more  than 
330  meters,  or  9.3  meters  per  kilometer.  Mr.  Lippincott  says  that  from 
1896  to  1900,  inclusive,  five  years,  there  was  practically  no  rainfall,  and 

"Leverett,  Frank,  The  water  resources  of  Illinois:  Seventeenth  Ann.  Kept.  U.  S.  Geol.  Survey, 
pt.  2,  1896,  pp.  805-806,  811. 

ftDarton,  N.  H.,  Artesian  waters  of  tlie  Dakota*:  Seventeenth  Ann.  Kept,  U.  S.  Geol.  Survey, 
pt.  2,  1896,  pp.  665-670,  pi.  Ixx. 


FLOW  AGE  IN  SUBCAPILLAKY  OPENINGS.  143 

therefore  no  addition  to  the-  ground  waters  During-  that  time  the  water 
table  fell  in  the  granitic  sand,  on  an  average,  at  the  rate  of  0.38  meter  per 
kilometer  per  annum.  This  fall  of  water  during  these  years  in  the  granitic 
sands  alone,  Mr.  Lippincott  says,  i.s  sufficient  to  account  for  the  entire  dis- 
charge of  the  Los  Angeles  River.  A  head  of  9.4  meters  per  kilometer  in 
large  channels  where  friction  is  small  would  result  in  the  outpouring  of  the 
great  quantity  of  water  held  in  the  gravels  into  the  Los  Angeles  River  in  a 
very  short  time.  But  the  openings  in  the  sands  are  capillary,  and  the  resis- 
tance due  to  friction  and  to  viscosity  is  such  that  the  water  was  very  slowly 
delivered  to  the  river  under  a  head  of  9.4  meters  per  kilometer,  the  average 
fall  being,  as  explained,  0.38  meters  per  kilometer  per  annum. 

Movement  as  slow  as  this  must  be  rapid  as  compared  with  the  exceed- 
ingly slow  movement  of  the  ground  water  in  the  artesian  basins  referred 
to.  It  follows  from  these  illustrations  that  the  ordinary  rates  of  movement 
in  the  belt  of  cementation  are  very  much  slower  than  were  the  move- 
ments under  the  conditions  in  which  Poiseuille,  King,  and  others  earned 
on  their  experiments.  It  is  plain  that  the  laws  derived  from  experiments 
as  given  by  Poiseuille  and  King  in  reference  to  capillary  flow  are  only 
very  partially  applicable  to  movements  of  ground  water;  indeed,  their 
application  is  probably  limited  to  the  somewhat  rapid  movements  of  the 
water  in  the  capillary  tubes  above  the  level  of  ground  water  in  the  belt  of 
weathering  where  gravity  has  its  full  effectiveness,  and  adjacent  to  large 
openings,  either  natural  or  artificial. 

subcapiiiary  openings. — By  subcapillaiy  openings,  as  already  explained,  are 
meant  openings  smaller  than  capillary  openings.  In  subcapiiiary  openings 
the  attraction  of  the  solid  molecules  extends  from  wall  to  wall,  and  there- 
fore in  these  openings  the  water  is  wholly  that  of  the  films  attached  to  the 
walls  by  molecular  attraction.  There  is  no  free  water,  in  the  sense  that 
the  molecules  are  free  to  move  among  themselves,  resisted  only  by  the  vis- 
cosity of  the  fluid.  The  ratio  of  the  resistance  to  movement  of  water  thus 
attached  as  films  to  solids  is  almost  infinitely  great  as  compared  with  that 
of  free  molecules.  Water  thus  attached  is  as  if  glued  to  the  walls. 

Quiucke  has  determined  that  the  attractive  influence  of  glass  upon  a 
fluid  extends  through  a  silver  film  0.00005  mm.  thick;  or,  stated  in  another 
way,  he  finds  that  the  distance  through  which  molecular  attraction  acts  is 


144  A  TREATISE  OM  METAMORPHISM. 

in  general  0.00005  mm."  Plateau  made  the  distance  through  which  mole- 
cular attraction  acts  p,^  mm.,6  which  amount  is  slightly  greater  than 
Quincke's  determination.  Since  each  wall  holds  a  film  of  water,  sheet  pas- 
sages below  0.0001  mm.  in  diameter  are  subcapillary.  The  maximum  size 
for  the  subcapillary  circular  openings  is  twice  as  great,  or  0.0002  mm.  in 
diameter. 

The  laws  of  flowage  of  water  through  tubes  of  such  small  size  have 
not  been  investigated,  so  far  a's  I  am  aware.  However,  upon  theoretical 
grounds  one  would  expect  that  the  flow  would  be  exceedingly,  indeed 
indefinitely,  slow  even  as  compared  with  flow  in  capillary  tubes.  -This 
Anticipation  is  fully  justified  by  the  observed  facts  of  geology.  It  is  well 
known  that  natural  oil  and  gas  may  be  held  in  anticlinal  arches  and  domes 
for  long  periods  of  time,  even  when  under  great  pressure.  It  is  certain  in 
these  cases  that  the  escape  of  oil,  or  even  gas,  through  the  subcapillary 
openings  of  the  shales  is  slower  than  the  manufacture  of  these  products  in 
nature's  laboratory.  The  facts  as  to  the  retention  of  oil  and  gas  under 
shale  roofs  render  it  highly  probable  that  flow  in  subcapillary  openings 
is  so  slow  as  to  be  inappreciable  during  the  time  through  which  an  experi- 
ment is  ordinarily  continued;  but  the  flow  in  subcapillary  openings  during 
geological  periods  is  probably  of  great  consequence.  (See  pp.  892-904.) 

It  may  be  anticipated  that  the  slow  movement  of  water  in  subcapillary 
openings  is  greatly  influenced  by  change  of  temperature.  At  high  temper- 
atures the  viscosity  of  water  is  an  important  element  in  flow,  and  this 
rapidly  decreases  with  increasing  temperature.  That  water  gas  does  not 
obey  the  law  of  flow  of  liquids  in  subcapillary  tubes  is  shown  by  the 
experiment  of  Daubri-e,"  in  which  the  vapor  of  water  at  a  temperature  of 
160°  C.,  nnd  consequently  at  a  pressure  of  6  atmospheres,  passed  through 
a  layer  of  apparently  solid  rock  2  cm.  in  thickness,  and  gave  a  pressure  on 
the  other  side  of  1.9  atmospheres.  This  experiment  shows  beyond  all 
question  that  water  gas  under  high  pressure  and  temperature  does  not 
adhere  to  the  walls  sti-ongly,  and  has  such  a  small  viscosity  that  it  slowly 
but  surely  passes  through  subcapillary  openings.  However,  ground  water 
at  all  temperatures  below  the  critical  temperature  under  ordinary  conditions 

«Quincke,  M.,  Ueber  die  Entfernung  in  welcher  die  Molecularkrafte  der  Capillaritat  noch  wirk- 

sind:  Poggendorff,  Annalen,  vol.  138,  p.  402. 

&  Plateau,  J.,  Statique  des  liquids,  vol.  1,  1873,  p.  210. 

••Daubrt'-e,  A.,  Geologic  exp^rimentale,  Paris,  1879,  vol.  1,  pp.  236-238. 


FLOW  AGE  IN  SUBCAPILLARY  OPENINGS.         145 

is  held  by  the  pressure  in  the  form  of  a  liquid.  But  at  temperatures 
higher  than  365°  C.,  or  the  critical  temperature  of  water,  whatever  the 
pressure,  the  water  is  in  the  form  of  water  gas.  In  this  case  it  may  be 
supposed  to  have  a  much  greater  penetrating  power  than  in  the  form  of 
liquid,  since  it  can  not  be  considered  as  adhering  to  the  walls  of  the 
openings. 

Even  if  subcapillary  openings  be  very  small  and  the  flow  very  slow, 
it  does  not  follow  that  the  water  within  these  minute  openings  is  not  an 
agent  through  which  important  geological  work  is  accomplished.  The 
water  in  such  spaces  is  capable  of  taking  into  solution  the  substances  with 
which  it  is  in  contact,  of  depositing  material  from  solution,  of  reacting  upon 
the  substances  by  hydration;  in  short,  is  capable  of  performing  all  the 
transformations  which  freely  moving  water  is  able  to  accomplish.  Indeed, 
it  has  already  been  seen  that  all  transfers  of  material  between  water  and 
rock  must  take  place  through  the  fixed  films  of  water.  (See  p.  64.) 
The  transfer  of  material  in  subcapillary  openings  is  confined  to  short 
distances  because  there  is  no  free  circulating  water.  The  interchanges  of 
material  are  probably  slow,  except  between  adjacent  or  nearly  adjacent 
mineral  particles;  therefore  it  seems  highly  probable  that  a  given  volume 
of  water  in  the  subcapillary  openings  is  far  more  effective  in  transforming 
rocks  than  an  equivalent  volume  in  larger  openings.  The  same  reasoning 
applies  here  as  in  the  case  of  the  capillary  openings  as  compared  with 
supercapillary  openings.  The  surface  of  action  per  unit  volume  in  the 
subcapillary  tubes  is  vastly  greater  than  in  larger  openings.  As  shown 
on  pages  686-698,  the  above  conclusion  as  to  the  efficacy  of  water  in 
subcapillary  openings  is  fully  justified  by  the  facts.  It  is  there  seen 
that  the  minute  amount  of  water  contained  in  the. subcapillary  openings  is 
the  medium  through  which  the  complete  transformation  of  rocks  to  schists 
and  gneisses  has  been  accomplished.  I  therefore  conclude  that,  while  it 
is  probable  that  the  actual  flow  of  water  and  transfer  of  material  in 
subcapillary  openings  is  comparatively  slow,  it  is  certain  that  most 
profound  alterations  of  rocks  take  place  through  this  water  as  the  agent  of 
transformation. 

Subcapillary  openings  include  the  openings  of  mechanical  sediments 
the  particles  of  which,  if  spherical  and  of  uniform  size,  are  not  greater  than 
00012  mm.  in  diameter.  As  a  matter  of  fact,  many  of  the  openings  in 
MON  XLVII — 04 10 


146  A  TREATISE  ON  METAMORPHISM. 

which  a  portion  of  the  particles  are  larger  than  this  have  subcapillary 
openings,  since  the  larger  openings  are  occupied  by  grains  as  small  as  or 
smaller  than  the  above  dimensions.  The  great  majority  of  the  clays, 
shales,  and  slates  are  largely  composed  of  particles  smaller  than  0.0012 
mm.  in  diameter  and  their  openings  are  subcapillary.  Minute  openings 
between  the  grains  of  the  igneous  rocks  and  of  the  rocks  metamorphosed 
to  schists  and  gneisses  are  also  usually  subcapillary.  Where  practically 
all  of  the  openings  are  subcapillary,  whether  they  be  the  openings  of 
sedimentary,  igneous,  or  metamorphic  rocks,  such  rocks  constitute  practi- 
cally impervious  strata;  for  the  contained  water  is  in  fixed  films  held  by 
molecular  attraction,  and  the  circulation,  as  already  explained,  is  so  slow  as 
to  be  negligible  during  short  time  intervals. 

PERCENTAGE  OF  OPENINGS,  OB  PORE  SPACE. 

The  percentage  of  openings  in  the  rocks,  or  the  pore  space,  is  a  func- 
tion of  the  number  and  the  size  of  the  openings.  In  so  far  as  the  openings 
in  rocks  are  large  and  numerous,  there  is  a  large  pore  space.  It  has 
already  been  seen  (pp.  124-129)  that  the  absolute  amount  of  openings  in 
rocks,  as  shown  by  observation,  varies  from  a  small  fraction  of  1  per  cent  to 
over  50  per  cent.  The  larger  the  pore  space  the  more  favorable  the  condi- 
tions for  circulation,  but  since  the  variation  in  pore  space  is  so  great  it  is 
evident  that  the  flowage  of  water  dependent  upon  porosity  is  very  variable. 
Water  passes  readily  through  rocks  which  contain  much  pore  space ;  water 
does  not  flow  to  an  appreciable  extent  through  rocks  which  have  a  small 
fraction  of  1  per  cent  of  pore  space.  Other  factors  being  the  same,  and  the 
pore  space  of  the  same  character,  the  flowage  is  in  direct  ratio  to  the  amount  of 
pore  space. 

FORCES  PRODUCIX«  WATEB  CIRCULATION. 

The  forces  producing  circulation  of  ground  water  are  gravity,  heat, 
mechanical  action,  molecular  attraction,  and  vegetation.  The  dominant 
force,  upon  which  the  movement  of  ground  water  mainly  depends,  is  gravi- 
tative  stress. 

GraVity. — Gravity  ever  tends  to  pull  the  water  downward.  And  this 
never-ceasing  force  at  work  throughout  the  zone  of  water  circulation,  on 
the  average  continuously  carries  the  circulating  water  to  lower  levels.  This 
condition  of  affairs  is  analogous  to  the  work  of  gravity  in  earth  movements.0 

o  Van  Hise,  C.  R.,  Earth  movements:  Trans.  Wisconsin  Acad.  Sci.,  Arts,  and  Letters,  vol.  11.,  1898, 
pp.  465-516. 


GRAVITY  PROMOTES  UNDERGROUND  CIRCULATION.          147 

But  in  earth  movements  and  water  circulation  alike,  all  the  elements  of  the 
movement  must  be  taken  into  account.  The  downward  movement  of  a 
greater  mass  of  earth  or  water  may  result  in  the  upward  movement  of  a 
lesser  mass.  The  upward  movements  of  water  dependent  upon  downward 
movements  of  other  water  are  of  relatively  greater  importance  in  the  water 
circulation  than  are  the  upward  movements  of  rocks  consequent  upon 
downward  movements  of  larger  masses  of  material  in  earth  movements^ 

Indeed,  it  will  be  seen  that  commonly  the  circulation  of  a  system  of 
ground  water  in  the  belt  of  cementation  involves  both  downward-moving 
and  upward-moving  masses.  In  such  systems  of  ground-water  circulation 
gravity  is  effective  in  the  movement  in  proportion  to  the  head.  Head  is 
due  to  the  fact  that  the  water  entering  the  ground  at  a  certain  level,  after  a 
short  or  long  underground  journey,  issues  at  a  lower  level. 

Also  where  there  is  a  difference  in  the  density  of  the  two  columns  due 
to  difference  in  the  amount  of  material  held  in  solution,  gravity  promotes 
circulation  independently  of  head,  the  column  holding  more  salts  being 
pulled  down  and  the  lighter  column  driven  upward.  Probably  the  amount 
of  material  in  solution  is  usually  not  so  great  as  to  make  this  an  important 
factor  in  the  process,  but  in  salt  regions  it  may  be  important.  The  density  of 
the  water  of  the  sea  as  compared  with  fresh  water  is  1.02765  to  1.02795,"  and 
the  density  of  a  saturated  solution  of  sodium  chloride  at  4°  C.,  as  experi- 
mentally determined  by  Mr.  S.  H.  Ball,  is  1.2063.  Of  course  in  actual  cases 
such  differences  as  these  are  not  found,  for  both  columns  are  sure  to  have 
salts  in  solution ;  but  where  springs  empty  under  the  sea  the  first  case  iS 
approached.  In  such  instances,  the  increased  density  of  the  sea  water 
opposes  the  head  of  the  lighter  stream  of  relatively  pure  water. 

Heat. — Change  in  temperature  may  result  in  the  expansion  and  contrac- 
tion of  water,  and  such  changes  in  volume  necessarily  involve  some  move- 
ment. The  volume  of  water  varies  as  the  temperature.  Taking  the 
volume  of  water  at  4°  C.  as  1,  its  volume  at  50°  C.  is  1.0120,  at  75°  C.  is 
1.0258,  and  at  100°  C.  is  1.0432.6  Therefore  the  increase  in  the 
temperature  of  underground  water  may  increase  its  volume  and  lessen  its 
density  as  much  as  4  per  cent  without  exceeding  its  boiling  point  at  atmos- 
pheric pressure,  and  a  difference  in  the  density  of  two  columns  by  1  per 

°  Bischof,  Gustav,  Elements  of  chemical  and  physical  geology,  translated  by  Paul  and  Drummond, 
Harrison  <fe  Sons,  London,  vol.  1,  1854,  p.  97. 

6  Austin,  L.  W.,  and  Thwing,  C.  B.,  Exercises  in  physical  measurements,  Allyn  &  Bacon,  Boston, 
1895,  p.  151. 


148  A  TREATISE  ON  METAMORPHISM. 

cent  or  more  is  probably  not  uncommon.  Decrease  in  temperature  may 
correspondingly  increase  the  density  of  water. 

Gravity  and  heat — While  change  of  temperature  necessarily  involves  some 
movement,  its  chief  effect  in  water  circulation  is  as  a  force  subordinate  to 
gravity.  In  so  far  as  water  in  a  connected  descending  and  ascending 
system  is  warmer  at  its  point  of  issuance  than  it  was  when  it  joined  the  sea 
of  underground  water,  this  gives  gravity  an  effect  in  circulation  in  the 
same  direction  as  head.  This  is  consequent  upon  the  fact,  noted  above, 
that  the  density  of  water  varies  inversely  with  the  temperature. 

It  is  therefore  evident  that  in  columns  of  water  of  equal  length  the 
stress  of  gravity  is  greater  upon  the  column  having  the  lower  temperature. 
That  the  diffence  in  gravitative  stress  due  to  difference  in  temperature  may 
be  sufficient  to  produce  rapid  circulation  in  pipes  that  are  supercapillary  is 
shown  by  the  use  of  the  principle  in  the  hot-water  system  of  heating 
buildings.  Underground,  as  in  the  hot-water  system  of  heating,  heat  is  the 
energy  which  causes  the  water  to  expand,  and  gives  a  difference  in  density. 
When  heat  has  produced  a  difference  in  density  of  the  two  columns, 
gravity  is  the  force  which  inaugurates  and  maintains  the  circulation. 

It  is  believed  that  underground  circulation  may  be  promoted  to  an 
important  extent  by  difference  in  temperature  of  the  descending  and 
ascending  columns  of  water,  resulting  from  heat  abstracted  from  the  rocks 
due  wholly  to  their  normal  increment  of  temperature  with  depth.  Later  it 
will  be  shown  that  the  downward-moving  water  is  ordinarily  dispersed  in 
many  small  openings  and  moves  relatively  slowly;  therefore  it  may  be 
supposed  at  any  given  place  to  have  approximately  the  temperature  of 
the  rocks.  The  upward  movement  of  water,  on  the  contrary,  is  shown  to 
be  usually  in  the  larger  openings  and  relatively  rapid;  therefore  at  any 
given  place  its  temperature  is  probably  higher  than  is  normal  for  the  rocks 
at  that  depth.  The  result  is  a  difference  in  temperature  between  the  des- 
cending and  ascending  columns,  the  ascending  column  being  the  warmer. 

In  regions  where  volcanism,  or  mechanical  action,  or  both,  have 
recently  occurred,  the  difference  in  density  resulting  from  difference  in 
temperature  between  the  descending  and  ascending  columns  is  likely  to  be 
a  much  more  important  influence  in  the  circulation  of  the  ground  waters 
than  in  regions  where  the  difference  in  temperature  is  due  to  the  normal 
heat  of  the  rocks.  Such  a  region  is  the  Yellowstone  Park. 


HEAT  INFLUENCES  IN  UNDERGROUND  CIRCULATION.         149 

In  some  countries  the  issuing  waters  throughout  great  regions  are  very 
clearly  at  a  higher  temperature  than  the  entering  waters,  and  in  such 
regions  the  difference  in  temperature  must  be  a  very  important  factor  in 
the  underground  circulation.  In  such  cases  the  difference  in  temperature 
between  descending  and  ascending  waters  generally  results  from  a  combina- 
tion of  the  normal  increase  of  temperature  due  to  depth,  from  regional 
volcanism,  and  from  the  rocks  having  a  higher  temperature  than  normal 
because  of  recent  orogenic  movements.  An  excellent  illustration  of  such 
regions  is  the  Cordilleran  region  of  western  United  States.  (See  pp.  591- 
592.) 

As  already  noted,  the  expansion  of  water  with  increase  of  temperature 
is  considerable,  amounting  to  over  4  per  cent  betv/een  0°  and  100°  C.; 
ihat  is,  a  given  mass  of  water  occupies  a  volume  4  per  cent  greater  at  the 
latter  than  at  the  former  temperature.  In  other  words,  if  there  be  an 
average  difference  of  100°  C.  between  the  ascending  and  descending 
columns,  100  meters  of  the  downward-moving  water  balances  104  meters  oi1 
the  upward-moving  water.  If  we  suppose  the  descending  and  ascending 
columns  to  be  connected,  of  equal  height,  and  having  an  average  difference 
in  temperature  of  100°  C.,  this  would  be  equivalent  to  a  head  of  4  meters  per 
100  meters  for  the  entire  height  of  the  column.  Probably  the  difference  in 
temperature  between  two  columns  is  not  often  so  great  as  100°  C.,  but  if  it 
be  sufficient  to  give  a  difference  in  density  of  1  per  cent,  and  the  ascending 
and  descending  columns  be  the  same  length,  this  is  ample  to  give  a  stress 
sufficient  to  overcome  friction  and  viscosity,  and  give  a  decided  movement 
to  ground  water.  As  an  illustration  of  the  principle  may  be  mentioned  the 
water  power  of  the  sea  mills  of  Cephalonia,  which,  according  to  the  Crosbys, 
is  wholly  due  to  difference  in  temperature  between  the  descending  and 
ascending  waters.0 

Mechanical  action. — A  third  force  influencing  ground-water  circulation  is 
mechanical  action.  Earth  movements  may  close  or  partly  close  the 
openings  in  the  rocks,  and  in  this  process  squeeze  out  the  water,  as  in  the 
production  of  the  schists  and  gneisses  from  the  sedimentary  rocks.  If  the 
deformation  of  the  rocks  be  referred  to  their  ultimate  cause,  gravity,  even 
the  circulation  of  the  water  resulting  from  deformation  is  indirectly  due  to 
the  stress  of  gravity. 

"Crosby,  W.  F.,  and  Crosby,  W.  O.,  The  sea  mills  of  Cephalonia:  Tech.  Quar.,  vol.  9,  1896, 
pp.  6-23. 


150  A  TREATISE  ON  METAMORPHIS.M. 

Moi«uiar  attraction. — The  fourth  force  affecting  the  movement  of  ground 
water  is  molecular  attraction.  This  attractive  force  works  between  the 
particles  of  water  themselves  (cohesion)  and  between  the  particles  of  water 
and  rock  (adhesion). 

As  a  result  of  molecular  attraction  water  may  rise  against  gravity  in 
capillary  or  hair-like  openings,  thus  saturating  the  rocks  at  higher  altitudes 
than  it  would  were  it  not  for  this  cause;  it  may  creep  aloiig  the  walls  of 
the  openings  of  rocks  without  extending  from  wall  to  wall,  and  therefore 
without  saturating  the  rocks. 

The  rise  of  water  when  it  fills  capillary  openings  raises  the  free  surface 
of  water  above  the  normal  level.  This  rise  of  the  free  surface  is  explained 
by  the  attraction  between  the  water  and  the  walls,  and  the  attraction  of  the 
molecules  of  water  for  one  another.  The  strong  attraction  between  the 
surfaces  of  mineral  grains  and  water  has  already  been  alluded  to.  As  a 
result  of  this,  water  tends  to  rise  along  a  wall  or  tube.  This  is  dependent 
upon  the  fact  that  there  is  greater  attraction  between  the  molecules  of  rock 
and  water  (adhesion)  than  between  the  molecules  of  water  themselves 
(cohesion).  However,  the  molecular  attraction  between  the  particles  of 
water  is  very  great.  The  strength  of  the  surface  tension  of  a  film  of  pure 
water  is  dependent  upon  cohesion,  and  is  81.96173  dynes  per  square  centi- 
meter." When  a  molecule  is  surrounded  on  all  sides  by  free  water  the 
attractions  in  the  various  directions  equalize  one  another,  and  so  particles 
are  comparatively  free  to  move  However,  at  the  surface  the  upward  com- 
ponent of  the  attraction  is  zero;  hence  there  is  effective  tangential  and 
downward  attraction.  The  rise  of  the  water  along  the  walls  is  due  to 
adhesion.  As  a  result  of  this  attraction  a  film  of  water  is  drawn  along  the 
walls.  Because  of  the  attraction  of  cohesion  the  film  of  adherent  water 
draws  up  the  next  row  of  molecules  away  from  the  walls;  these  molecules 
in  turn  exert  an  attractive  force  on  the  next  adjacent  molecules,  and  so  on. 
The  attractive  force  of  the  surface  film  of  water  for  the  water  below  draws 
up  the  molecules  constituting  it;  this  in  turn  acts  upon  the  film  below,  and 
so  on.  The  total  effect  of  the  molecular  attraction  between  the  walls  and 

« Daniell,  Alfred,  A  text-book  of  the  principles  of  physics,  3d  ed.,  Macmillan  Co.,  New  York, 
1895,  pp.  271-279.  Ostwalrt,  W.,  Outlines  of  general  chemistry,  translated  by  James  Walker,  3d  ed., 
Macmillan  Co.,  New  York,  1895,  pp.  107-111.  Barker,  Geo.  F.,  Physics,  Holt  &  Co.,  New  York,  1892, 
pp.  200-211. 


MOLECULAR  ATTRACTION  AND  GROUND  CIRCULATION.       151 

the  water  and  the  molecular  attraction  between  the  particles  of  water  is  to 
produce  an  elevation  above  the  normal  surface  of  the  water,  the  upper 
surface  of  which  is  of  a  shape  as  though  it  were  an  elastic  membrane 
adhering  to  the  walls  and  being  stretched  by  the  weight  of  the  water  above 
the  ordinary  level  below. 

The  height  to  which  water  rises  above  this  natural  level  is  indirectly 
as  the  diameter  of  the  capillary  openings.  In  circular  glass  tubes  1  mm.  in 
diameter,  at  20°  C.,  pure  water  rises  3.32  cm."  Between  plates  1  mm.  apart 
it  rises  half  of  this  amount.  Since  the  height  is  inversely  as  the  diameters 
of  the  openings,  in  circular  tubes  0.01  mm.  in  diameter,  the  height  in  tubes 
would  be  3.32  m.  and  in  sheet  openings  1.66  m. 

In  circular  tubes  0.001  mm.  in  diameter  the  height  in  tubes  would  be 
33.2  m.,  and  in  sheet  openings  16.6  .m.;  and  in  circular1  tubes  0.0002  mm.  in 
diameter — that  is,  openings  of  a  size  intermediate  between  subcapillary  and 
capillary — the  water  would  rise  to  a  height  of  166  in.,  and  in  sheet  open- 
ings 83  m.  Since  many  rocks  have  openings  as  small  as  or  even  smaller 
than  this,  capillary  attraction  may  be  very  important  in  the  position  of  the 
ground-water  level.  (See  pp.  411-412.)  If  the  openings  are  inclined  the 
lengths  of  the  openings  thus  filled  are  correspondingly  great. 

The  height  to  which  the  water  rises  is  independent  of  the  character  of 
the  walls,  provided  the  walls  are  wetted,6  and  hence  the  above  numbers  are 
applicable  to  rocks.  However,  the  height  to  which  the  water  rises  dimin- 
ishes as  the  temperature  increases;  hence,  the  above  numbers  should  be 
modified  somewhat  as  the  top  of  the  sea  of  ground  water  has  a  temperature 
below  or  above  a  temperature  of  20°  C.  Ordinarily  this  modification  is  of 
minor  importance. 

Above  the  level  to  which  the  water  may  be  raised  as  a  continuous  sheet 
in  the  capillary  openings,  the  water  may  still  creep  along  the  walls  of  the 
openings  without  filling  them.  The  obstinacy  with  which  a  film  of  water 
holds  to  the  rock  surface  has  already  been  explained.  This  water  is  that  of 
imbibition  (p.  124).  In  proportion  as  the  water  of  imbibition  varies  in 
amount  the  water  under  molecular  attraction  creeps  from  areas  of  greater 
humiditv  to  areas  of  less  humidity.  To  the  rise  of  the  free  surface  due  to 
capillarity  there  is  a  definite  limit;  there  is  no  limit  to  the  creep  of  water 
along  the  walls.  It  is  presumable,  however,  that  such  movement  is  rela- 

a  Barker,  cit.,  pp.  209-210.  &  Barker,  cit.,  p.  210. 


152  A  TREATISE  ON  METAMORPHISM. 

tively  slow,  and  that  the  amount  of  water  which  is  thus  transferred  for 
a  given  surface  is  small.  But  in  the  soils  very  large  surfaces  are  available 
for  creep,  and  therefore  this  process  is  a  very  important  One,  especially 
in  connection  with  plant  growth.  The  process  is  one  which  especially 
pertains  to  the  belt  of  weathering  and  is  therefore  later  considered.  (See 
pp.  412-423.) 

The  rise  of  the  free  surface  of  ground  water  above  the  normal  level 
saturating  the  rocks,  the  creep  of  water  along  the  walls  without  saturation, 
and  the  flowage  of  water  through  small  tubes  where  there  is  no  free  sur- 
face are  generally  described  under  the  term  capillarity.  However,  it  is 
evident  that  under  the  term  thus  used  are  included  three  very  different 
things.  The  principles  involved  in  the  flow  of  water  through  capillary 
tubes  are  very  different  from  those  which  control  the  free  surface  of  ground 
water  in  capillary  tubes,  and  these  laws  again  are  different  from  those 
which  control  the  creep  of  water  along  the  walls  of  openings. 

vegetation — The  roots  of  plants  absorb  ground  water  and  transport  it  to 
the  surface.  'The  absorption  of  water  by  plant  roots  causes  a  relative 
deficiency  of  water.  This  deficiency  is  remedied  by  the  movement  of 
water  from  other  places  toward  the  roots  by  the  forces  already  considered. 
But  the  influence  of  roots  upon  the  flow  of  ground  water  mainly  concerns 
the  belt  of  weathering.  The  subject  is  therefore  later  considered.  (See 
pp.  417,  422-423.) 

General  statements. — In  conclusion,  it  may  be  said  that  the  immediate  cause 
of  movements  of  ground  water  are  five — gravity,  heat,  mechanical  action, 
molecular  attraction,  and  vegetation. 

So  far  as  the  forces  are  concerned,  the  vertical  component  of  the  move- 
ments of  ground  water  is  of  far  the  greatest  importance. 

But  whatever  the  cause  of  the  flow  of  ground  water,  the  direction  of 
movement  is  from  places  of  greater  pressure  to  places  of  less  pressure.  A 
current  going  in  any  direction  is  evidence  of  an  excess  of  pressure  in  the 
rear  of  the  current.  Thus  water  which  enters  by  seepage  or  through  capil- 
lary tubes  into  a  larger  opening,  such  as  a  fissure,  must  be  under  greater 
pressure  than  the  column  of  water  into  which  it  makes  its  way.  Whether 
the  motive  force  in  the  movement  of  the  water  be  difference  in  gravitative 
stress  or  temperature,  or  any  other  cause,  the  excess  of  pressure  resulting 
in  movement  is  behind  the  current. 


VISCOSITY  RETARDS  UNDERGROUND  CIRCULATION.          153 

THE  FACTOR  OPPOSIXii  WATER  CIRCULATION. 

The  factor  opposing  water  circulation  is  internal  friction  of  the  water. 
The  internal  friction  is  dependent  upon  the  viscosity  of  the  solutions.  The 
elements  entering  into  viscosity  are  the  concentration  of  the  solutions  and 
the  temperature.  The  more  concentrated  the  solutions  the  greater  the 
viscosity ;  but  as  the  underground  solutions  of  water  are  commonly  not 
strong,  this  is  ordinarily  not  an  important  element.  The  viscosity  of  water 
decreases  very  rapidly  with  increase  of  temperature.  The  relative  viscosity 
of  pure  water  at  0°  C.,  45°  C.,  and  90°  C.  is  respectively  100.00,  33.89,  and 
18.16.  (See  p.  141.)  From  these  ratios  it  is  apparent  that  the  viscosity  of 
water  at  45°  C.  is  about  one-third  of  that  at  0°  C.,  and  at  90°  C.  only  about 
one-fifth  of  that  at  0°  C. 

It  is  therefore  clear  that  the  higher  the  temperature  the  less  the 
viscosity  and  the  less  the  internal  friction.  Internal  friction  due  to  viscosity 
results  from  the  variable  speeds  of  different  parts  of  moving  water  columns 
and  from  the  friction  between  the  moving  and  fixed  portions.  The  greater 
the  variations  in  speed  of  the  moving  parts  the  greater  the  internal  friction 
due  to  this  cause. 

Water  usually  wets  the  surface  of  the  rocks.  In  other  words,  there  is 
molecular  attraction  between  the  water  solutions  and  the  minerals  com- 
posing the  rocks.  This  attraction  is  so  strong  that  a  thin  film  of  water  is 
firmly  held  by  the  walls  of  the  openings — so  firmly  that  it  may  be  consid- 
ered as  fixed;  at  least  the  only  interchange  which  occurs  between  it  and 
the  passing  water  currents  is  that  of  diffusion,  not  that  of  flow.  Daniell 
says  the  friction  between  the  layer  of  adherent  water  and  the  rock  is 
infinite  as  compared  with  the  friction  within  the  liquid."  That  the  friction 
is  between  the  moving  liquid  and  the  fixed  film  of  liquid  is  shown  by  the 
fact  that  for  any  liquid  the  composition  of  the  walls  has  no  effect  upon  the 
flowage.6  This  being  the  case,  it  is  clear  that  in  the  flowage  of  water 
through  tubes  there  is  no  friction  of  the  water  against  the  rock  walls.  The 
adherent  films  of  water  are  the  walls  of  the  moving  columns,  and  the  internal 
friction  between  the  water  and  the  walls  is  that  between  the  fixed  films  and 
moving  water. 

"Daniell,  Alfred,  A  text-book  of  the  principles  of  physics,  3d  ed.,  Macuiillan  Co.,  New  York, 
1895,  p.  306. 

t>  Daniell,  cit,  p.  316. 


154  A  TREATISE  ON  METAMORPHISM. 

The  greater  the  speed  of  the  moving  water  the  greater  the  internal 
friction,  because  of  the  differential  movements  both  in  the  moving  water 
and  between  the  moving  water  and  the  films  fixed  to  the  walls.  Where 
the  rate  of  movement  is  sufficiently  slow  the  internal  friction  due  to  viscosity 
drops  to  an  almost  inappreciable  factor.  Therefore  where  the  movement 
is  very  slow,  even  if  the  passages  be  long  and  small,  the  pressure  due  to 
head  may  dimmish  very  slowly.  Indeed,  nearly  the  full  pressure  may  be 
maintained  for  long  distances — many  or  even  hundreds  of  kilometers. 
This  principle  is  of  the  utmost  importance  in  the  flowage  of  ground  water, 
and  its  applications  are  later  developed.  (See  pp.  585-588.) 


(JKXEKAIi  STATKMKXTS. 


Ill  general  it  may  be  said  that  in  proportion  as  the  driving  forces, 
gravity,  mechanical  action,  etc.,  are  great,  circulation  is  likely  to  be  rapid. 
In  proportion  as  the  opposing  force,  internal  friction,  is  great,  circulation  is 
likely  to  be  slow.  In  proportion  as  the  openings  approach  the  circular 
form,  circulation  is  likely  to  be  rapid.  In  proportion  as  the  openings  are 
continuous,  the  circulation  is  likely  to  be  rapid.  In  proportion  as  the  pore 
space  is  great,  circulation  is  likely  to  be  rapid. 

However,  of  all  these  various  factors  dependent  upon  the  character  of 
the  openings,  that  of  size  is  probably  the  most  important;  for  rocks  which 
do  or  do  not  readily  transmit  water  may  have  the  same  proportion  of  pore 
space.  For  instance,  if  the  grains  be  supposed  to  be  spherical,  of  the  same 
size,  and  arranged  in  the  most  compact  fashion  possible,  the  unoccupied 
space  is  0.26  of  the  entire  space,  without  reference  to  the  size  of  the  grains. 
Thus  the  relative  proportion  of  the  openings  in  a  great  bowlder  conglom- 
erate and  a  fine-grained  clay  may  be  the  same.  But  the  capacity  for  the 
transmission  of  water  by  the  former  will  be  indefinitely  greater  than  by  the 
latter.  As  illustrating  this,  an  experiment  showed  that  a  quartz  sand,  the 
water  of  saturation  of  which  was  the  same  as  that  of  a  certain  chalk,  trans- 
mitted water  under  a  certain  pressure  six  hundred  times  as  fast  as  the  chalk." 
In  the  compact  soils,  the  particles  of  which  are  exceedingly  small  (see 
pp.  138-146),  the  openings  between  the  particles  are  of  capillary  or  sub- 

a  Prestwich,  Joseph,  Geology,  chemical,  physical,  and  s,tratigraphical,  Clarendon  Press,  Oxford, 
vol.  1,  1886,  p.  159. 


PORE  SPACE  AND  UNDERGROUND  CIRCULATION.  155 

capillary  size.  In  the  case  of  the  fine  soils  and  clays  the  pores  may  be  almost 
wholly  subcapillary,  or  the  water  is  that  of  imbibition.  In  this  fact  we 
have  the  explanation  of  the  retention  of  soil  moisture  in  fine  clays.  The 
moisture  is  glued  to  the  grains.  There  is  practically  no  circulation,  and 
the  water  is  removed  only  by  high  temperature  or  high  pressure,  or  the 
two  combined.  It  follows  from  the  foregoing  that,  under  given  conditions 
with  a  given  pore  space,  the  coarse  conglomerates,  furnish  a  much  larger 
flow  than  fine  conglomerates,  the  fine  conglomerates  a  larger  flow  than  the 
sandstones,  and  these  a  vastly  greater  flow  than  the  soils,  clays,  and  shales. 

Bedding,  fault,  joint,  and  fissility  openings  may  be  so  close  together 
that  the  pore  space  is  very  large.  Ordinarily  fault  openings  are  wider 
spaced  but  larger  than  the  joint  openings,  and  joint  openings  are  wider 
spaced  and  larger  than  the  openings  of  fissility.  It  can  not  be  said  which 
kind  of  opening  gives,  on  the  average,  the  larger  pore  space.  Since, 
however,  large  openings  are  favorable  to  rapid  flow,  for  a  given  pore  space 
the  fault  openings  are  likely  to  give  a  greater  flow  than  joint  openings,  and 
joint  openings  a  greater  flow  than  those  of  fissility.  This  follows  from  the 
greater  size  of  the  fewer  openings.  To  this  is  to  be  added  the  element  of 
greater  continuity  of  the  larger  openings,  as  explained  on  pages  130- 
131.  Therefore,  with  a  given  pore  space  the  flow  may  be  vastly  greater 
in  the  case  of  faults  than  in  the  case  of  joints,  and  much  greater  in  the 
case  of  joints  than  in  the  case  of  fissility. 

In  this  connection  it  may  be  said  that  the  capacity  of  a  rock  for 
imbibition  gives  a  very  good  idea  as  to  its  power  of  transmission.  The 
water  of  imbibition,  it  may  be  recalled  (see  p.  124),  is  the  amount  which 
adheres  to  the  walls  of  the  openings.  It  is  evident  that  in  rocks  containing 
the  same  percentage  of  water  when  saturated  the  power  of  transmission 
varies  inversely  as  their  capacity  for  imbibition.  If  the  openings  of  a  rock 
be  very  small,  but  numerous,  there  is  in  a  cubic  centimeter  a  large  surface 
to  which  the  water  can  adhere.  If  the  openings  be  subcapillary,  the  water 
of  imbibition  and  saturation  are  the  same  and  the  powers  of  transmission 
practically  nil.  If  the  spaces  be  capillary,  the  water  of  imbibition  is  much 
less  and  the  power  of  transmission  greatly  increased.  If  the  spaces  be 
supercapillary,  the  water  of  imbibition  is  slight  in  amount  and  the  power 
of  transmission  very  great. 


156  A  TREATISE  ON  METAMORPHISM. 

As  already  noted,  there  are  two  zones  of  metamorphism,  that  of  kata- 
morphism  and  that  of  anamorphism,  and  the  former  consists  of  a  belt  of 
weathering  and  a  belt  of  cementation. 

The  major  part  of  the  water  entering  the  ground  must  finally  reach  the 
surface.  A  small  part  may  be  combined  with  the  rocks  in  the  underground 
course  of  the  water.  A  small  part  may  possibly  penetrate  deep  within  the 
zone  of  anamorphism,  but  it  is  safe  to  say  that  at  least  99  per  cent  of  the 
water  entering  the  ground  reappears  at  the  surface  in  some  manner.  A 
very  large  part  of  the  water  penetrating  the  soil  is  drawn  to  the  surface 
after  having  taken  a  longer  or  shorter  journey  in  the  belt  of  weathering.  A 
lesser  part  of  the  water  joins  the  sea  of  ground  water  and  takes  a  journey 
of  greater  or  less  distance  in  the  belt  of  cementation  before  it  reaches  the 
surface.  This  journey  may  be  merely  from  the  top  of  a  small  hill  to  its 
base,  or  it  may  be  hundreds  of  kilometers.  An  exceedingly  small  fraction 
of  the  water  doubtless  penetrates  the  zone  of  anamorphism,  although,  as 
explained  (pp.  665-668),  the  general  movement  is  from  rather  than  to  this 
zone.  The  underground  journeys  of  water,  whether  the  exceedingly 
short  ones  within  the  belt  of  weathering  or  the  longer  journeys  in  the  belt 
of  cementation  or  the  zone  of  anamorphism,  may  be  resolved  into  two 
components,  one  parallel  to  the  surface  of  the  earth  and  one  at  right 
angles  to  this  surface.  The  first  may  be  called  the  horizontal  component, 
the  second  the  vertical  component.  On  the  average,  the  horizontal  com- 
ponent of  the  journey  is  many  times  longer  than  the  vertical  component. 

GEOLOGICAL  WORK  OF  GROUND  WATER. 

From  the  foregoing  it  follows  that  the  geological  work  of  ground 
water  is  favored  by  smallness  of  openings,  by  length  of  time,  by  pressure, 
and  by  high  temperature.  Water  enters  the  rocks  mainly  through  the 
smaller  openings.  A  very  large  surface  of  the  rock  material  is  exposed  to 
water  action.  In  so  far  as  the  water  passes  from  the  smaller  openings  to 
the  larger  openings  its  geological  work  is  lessened.  The  geological  work 
may  be  considered  as  directly  proportional  to  the  time.  The  smaller  the 
openings  the  greater  the  resistance,  and  therefore  the  greater  the  time  for  a 
given  journey.  That  the  resistance  runs  up  very  rapidly  as  the  openings 
become  small,  and  especially  as  they  become  capillary  or  subcapillary,  has 


GEOLOGICAL  WORK  OF  GROUND  WATER.        157 

already  been  shown.  Since  the  horizontal  journey  is,  on  the  average,  long 
as  compared  with  the  vertical  journey,  the  element  of  time  is  of  much 
greater  importance  in  the  horizontal  component  of  the  journey  than  in  the 
vertical  component.  The  capacity  for  geological  work  is  increased  by 
pressure  and  by  temperature.  These  forces,  under  ordinary  conditions,  are 
a  function  of  depth,  and  these  factors  in  the  work  mainly  concern  the  ver- 
tical component  of  movement.  During  the  downward  journey  the  pressure 
and  temperature  steadily  increase,  and  the  amount  of  material  in  solution 
increases.  During  the  upward  journey  the  pressure  and  temperature 
diminish  and  the  tendency  for  material  to  pass  from  solution  or  to  be 
precipitated  increases,  and  the  amount  held  in  solution  diminishes. 

Pressure  and  temperature  are  ever  working  together  according  to 
definite  laws.  Both  increase  in  efficiency  with  depth,  and  they  greatly 
promote  the  activity  of  deep  ground  waters.  However,  of  all  the  vary- 
ing factors,  varying  temperature  is  the  one  which  is  of  incomparably 
the  greatest  importance.  High  temperature  ordinarily  results  from  depth 
of  penetration;  but  it  has  been  pointed  out  that  it  may  result  from  various 
other  causes,  of  which  chemical  action,  mechanical  action,  and  the  pres- 
ence of  intrusive  igneous  rocks  are  the  more  important.  The  capacity 
which  water  has  for  taking  and  holding  in  solution  various  relatively 
insoluble  compounds,  and  the  velocity  of  chemical  reactions,  increase 
enormously  with  increase  of  temperature.  Not  only  is  high  temperature 
favorable  to  geological  work,  because  of  the  chemical  activity  of  the  water, 
but,  as  already  pointed  out,  high  temperature  greatly  decreases  its  viscosity, 
and  this,  as  already  explained,  is  favorable  to  depth  of  penetration  and 
flow  through  minute  openings.  Since  the  temperature  changes  of  ground 
water  are  commonly  dependent  upon  depth,  the  vertical  component  of  the 
movement  of  underground  water  is  ordinarily  far  more  important  than  the 
longer  horizontal  component. 

The  underground  journey  of  water  may  occupy  hundreds  of  years. 
(See  pp.  585-586.)  The  surface  of  contact  in  very  small  openings  is 
very  great.  Under  these  conditions  of  slow  movement  and  small  openings 
there  is  sufficient  time  nearly  to  establish  complete  equilibrium  between 
the  solutions  and  the  solids  with  which  they  are  in  contact;  but  it  has  been 
seen  (pp.  34-35)  that  rarely  or  never  is  the  adjustment  of  a  rock  to  its 


158  A  TREATISE  ON  METAMORPHISM. 

environment  complete.  In  so  far  as  the  adjustment  is  not  complete,  changes 
are  going  on,  and  the  conditions  are  everywhere  those  of  chemical  dynamics, 
although  the  chemical  action  may  be  so  slow  that  if  the  operations  were 
conducted  in  a  laboratory  it  might  be  concluded  that  the  conditions  were 
those  of  chemical  statics.  This,  however,  but  illustrates  the  importance  of 
time  in  geological  operations. 

Thus  far  the  treatment  of  the  circulation  and  work  of  ground  water 
has  been  general.  There  are  many  other  factors  concerned  in  the  circula- 
tion which  have  not  yet  been  considered,  but  these  are  factors  special  to 
the  different  belts  and  zones.  They  will  therefore  be  treated  in  Chapters 
VI,  VII,  and  VIII,  on  the  belt  of  weathering,  the  belt  of  cementation,  and 
the  zone  of  anamorphism,  respectively. 


CHAPTER  IV. 

THE  ZONES  AND  BELTS  OF  METAMORPHISM. 

GENERAL  CONSIDERATIONS. 

The  various  geological  factors  which  bear  upon  metamorphism  have 
been  briefly  discussed  in  the  introductory  chapter.  It  is  there  held  that 
the  geological  factor  of  dominating  importance  is  depth.  Upon  the  basis 
of  depth  it  is  stated  that  the  known  crust  of  the  earth  is  divisible  into  upper 
and  lower  zones  of  metamorphism;  the  first  is  called  the  zone  of  katamor- 
phism,  and  the  second  the  zone  of  anamorphism.  It  is  further  stated  that 
the  zone  of  katamorphism  is  divisible  into  two  belts,  an  upper  belt  of 
weathering  and  a  lower  belt  of  cementation. 

While  in  the  introductory  chapter  these  general  statements  were  made, 
there  was  no  attempt  to  show  that  they  are  correct.  It  is  one  of  the  pur- 
poses of  this  and  the  following  chapters  to  furnish  evidence  of  the  utility 
of  this  classification,  and  to  show  that  very  different  metamorphic  effects 
follow  from  the  work  of  the  same  forces  and  agents  in  the  different  belts  and 
zones.  In  the  present  chapter  a  brief  general  statement  will  be  made  as  to 
the  characteristic  reactions  of  the  different  zones  and  belts.  This  statement 
is  primarily  from  physical  and  chemical  points  of  view.  The  next  chapter 
will  treat  of  the  alterations  of  minerals  with  reference  to  the  different  zones 
and  belts.  In  succeeding  chapters  the  alterations  of  the  rocks  in  the  belt 
of  weathering,  the  belt  of  cementation,  and  the  zone  of  anamorphism  will 
be  taken  up  in  detail.  The  treatment  will  be  primarily  from  the  geological 
point  of  view,  but  with  reference  to  physical  and  chemical  principles. 
Finally,  the  alterations  of  the  individual  rocks  will  be  considered.  This 
and  the  following  chapters  might  be  regarded  as  a  consideration  of  the 
metamorphism  of  the  crust  of  the  earth  from  the  point  of  view  of  the 
physical-chemical  principles  developed  in  Chapters  II  and  III. 

159 


160  A  TREATISE  ON  METAMORPHISM. 

It  has  just  been  stated  that  the  nature  of  the  metamorphism  varies 
greatly  with  depth.  The  physical  reasons  for  this  are  that,  as  depth 
increases,  temperature  and  pressure  increase.  It  has  been  seen  in  Chapters 
II  and  III  that  where  the  pressure  is  moderate  chemical  reactions  are  likely 
to  be  such  that  heat  is  liberated,  and  this  is  a  fact  whether  the  reactions 
decrease  or  increase  the  volume.  It  has  also  been  seen  that  where  the 
pressure  is  great  this  is  likely  to  be  the  controlling  factor,  and  that  under 
such  circumstances  reactions  take  place  which  lessen  the  volume  of  the 
materials.  Whether  the  reactions  take  place  with  liberation  of  heat  or 
with  absorption  of  heat  is  a  subordinate  matter;  but  very  commonly  the 
reactions  are  of  a  kind  that  absorb  heat. 

When  the  law  of  chemical  affinity  controls,  and  the  reactions  take 
place  with  liberation  of  heat  irrespective  of  the  volume  change,  the  reac- 
tions may  be  said  to  be  chemical -physical  reactions.  Where  pressure  is  a 
dominant  factor  and  reactions  take  place  with  diminution  of  volume 
irrespective  of  the  heat  change,  the  reactions  may  be  said  to  be  physical- 
chemical.  It  is  because  variations  in  the  geological  factor  of  depth  result 
in  these  contrasting  reactions  that  the  lithosphere  is  divisible  into  a  zone  of 
katamorphism  and  a  zone  of  anamorphism. 

ZONE  OF  KATAMORPHISM. 

From  the  surface  of  the  earth  to  a  very  considerable  depth  below  the 
surface  (for  strong  rocks  possibly  10,000  or  12,000  meters  under  quiescent 
geological  conditions)  the  rocks  as  originally  formed  may  contain  many 
openings,  as,  for  instance,  those  of  sandstones,  vesicular  lavas,  etc.  Even  if 
not  originally  porous  deformation  may  fracture  the  rocks  and  thus  produce 
many  openings.  Where  the  rocks  contain  openings  chemical  reactions  may 
take  place,  increasing  the  volume  of  the  material  without  rupturing  the 
rocks  and  without  raising  them  to  a  higher  position.  In  the  outer  litho- 
sphere  the  pressures  and  temperatures  are  moderate.  Under  such  circum- 
stances the  reactions  which  take  place  are  controlled  mainly  by  the  laws 
of  chemical  affinity,  not  by  the  influence  of  pressure.  At  low  temperatures 
the  fundamental  chemical  law  is  that,  on  the  whole,  the  preponderating 
chemical  reactions  are  those  which  take  place  with  the  liberation  of  heat  in 
accordance  with  the  first  part  of  van't  Hoffs  law.  Therefore  in  this  zone  the 
occurrence  of  a  reaction  in  the  alteration  of  a  rock  is  favorable  to  further 


REACTIONS  IN  ZONE  OF  KATAMORPHISM.  161 

alteration;  for  the  heat  developed  by  the  first  reaction  is  retained  by  the 
adjacent  material,  at  least  for  a  time,  and  this  promotes  further  reaction,  etc. 
But  this  tendency,  as  has  been  seen,  may  be  reversed  if  the  temperature 
becomes  too  high.  (See  p.  79.) 

Since  the  law  of  chemical  reactions  with  the  liberation  of  heat  is  the 
dominant  factor  in  this  upper  zone,  alterations  may  take  place  which 
work  with  or  against  pressure.  In  the  first  case  both  the  chemical  reaction 
and  the  compression  in  volume  result  in  the  liberation  of  heat.  In  the 
second  case  the  heat  liberated  is  that  developed  by  the  chemical  reaction 
minus  that  absorbed  as  a  result  of  the  work  done  in  expanding  the  volume. 

As  a  matter  of  fact,  near  the  surface  of  the  earth  the  very  important 
reactions  from  the  point  of  view  of  the  nonmetallic  elements,  aside  from 
solution,  are  those  of  oxidation,  hydration,  and  carbonation.  Oxidation 
and  hydration  commonly  involve  the  addition  of  material,  although  the 
former  frequently  occurs  by  substitution  of  oxygen  for  sulphur,  and 
therefore  by  desulphidation.  Carbonation  frequently  involves  the  addition 
of  material,  but  more  commonly  occurs  by  the  substitution  of  CO2  for  SiO2 
and  the  decomposition  of  silicates.  Often  the  freed  silica,  or  a  part  of  it, 
remains  in  situ.  All  of  these  reactions  are  well  known  to  liberate  heat. 
Commonly  they  decrease  rather  than  increase  the  specific  gravity  of  the 
minerals.  Since  they  usually  involve  addition  of  material,  it  is  clear  that 
where  all  the  residual  material,  or  a  large  part  of  it,  remains  in  situ  the 
volume  of  the  rocks  is  considerably  increased.  However,  it  will  be  seen 
that  solution  is  also  a  very  important  reaction  in  parts  of  the  zone  of 
katamorphism,  and  where  this  takes  place  to  a  sufficiently  great  extent 
the  volume  of  material  may  be  decreased. 

The  main  part  of  the  oxygen  and  much  of  the  carbon  dioxide  for 
oxidation  and  carbonation  is  directly  or  indirectly  derived  from  the  atmos- 
phere. The  water  is  chiefly  that  of  the  ground  circulation.  It  is  there- 
fore clear  that  in  the  upper  zone  oxygen  and  carbon  dioxide  are  being 
steadily  abstracted  from  the  atmosphere  and  fixed  in  the  rocks,  and  ground 
water  is  steadily  becoming  fixed  by  hydration.  The  amount  of  oxygen 
and  carbon  dioxide  thus  fixed  is  great.  If  it  were  not  for  replenishment, 
it  is  little  short  of  certain  that  the  carbon  dioxide  of  the  atmosphere 
would  have  long  since  become  exhausted.  But  probably  the  amount  of 
water  fixed  by  hydration  is  even  greater  than  that  of  the  gases,  oxygen 
MON  XLVII — 04 11 


162  A  TREATISE  ON  METAMOKPHISM. 

and  carbon  dioxide.  Analyses  of  rocks  in  the  upper  zone  of  metamorphism 
show  that  the  amount  of  combined  water  runs  as  high  as  4.42  per  cent  in 
shales  (see  p.  744),  and  it  probably  averages  as  high  as  1.64"  per  cent. 
When  it  is  remembered  that  the  zone  of  katamorphism  extends  to  a  depth 
of  thousands  of  meters,  it  is  apparent  that  the  amount  of  water  which  is 
thus  fixed  in  the  rocks  by  the  process  of  hydration  is  enormous.  However, 
it  will  be  seen  that  the  process  of  hydration,  like  that  of  carbonation,  is 
reversed  in  the  zone  of  anamorphism. 

By  the  statement  that  oxidation,  carbonation,  and  hydration  are  the 
very  important  characteristic  reactions  of  the  zone  of  katamorphism  it  is 
not  meant  to  imply  that  the  reverse  reactions  do  not  take  place  to  some 
extent.  In  fact,  deoxidation,  decarbonation,  and  dehydration  all  occur; 
but  oxidation,  carbonation,  and  hydration  are  greatly  preponderant,  and 
indeed  dominant  over  the  reverse  reactions. 

Summarizing  so  far  as  the  energy  factors  are  concerned,  the  changes  in 
volume  commonly  absorb  heat,  the  chemical  reactions  dominantly  liberate 
heat  and  only  exceptionally  absorb  heat.  The  heat  liberated  by  the  chem- 
ical reactions  is  certainly  very  much  greater  than  the  sum  of  that  absorbed 
by  the  volume  changes  and  that  absorbed  by  the  exceptional  chemical 
reactions.  Therefore,  so  far  as  the  rocks  of  the  zone  of  katamorphism  are 
concerned,  the  total  of  the  volume  and  chemical  changes  results  in  the 
liberation  of  heat  and  the  dissipation  of  energy. 

The  minerals  formed  in  the  zone  of  katamorphism  are  comparatively 
few  in  number,  with  low  specific  gravities  and  probably  for  the  most  part 
comparatively  simple  molecules;  hence  the  propriety  of  calling  this  zone 
the  zone  of  katamorphism,  or  katamorphic  zone.  This  use  of  the  term 
katamorphism  is  parallel  to  the  use  of  the  term  katabolism  in  biology  to 
designate  those  chemical  changes  within  a  living  body  which  result  in  the 
production  of  simple  compounds  from  more  complex  ones.  The  zone  of 
katamorphism  may  therefore  be  defined  as  the  zone  in  which  alterations  of 
rocks  result  in  the  production  of  simple  compounds  from  more  complex  ones. 

The  zone  of  katamorphism  is  divisible  into  two  belts,  (1)  an  upper 
belt  of  weathering,  and  (2)  a  lower  belt  of  cementation.  The  belts  are 

"This  is  the  average  taken  from  analyses  of  shales,  sandstones,  limestones,  and  volcanic  and 
crystalline  rocks,  given  by  F.  W.  Clarke  in  Bulls.  U.  S.  Geol.  Survey  No.  78,  pp.  36-37,  and  No.  168, 
pp.  16-17. 


EEACTIONS  IX  BELT  OF  WEATHERING.  163 

delimited  by  the  level  of  ground  water.  The  separation  of  the  belt  of 
weathering  from  the  belt  of  cementation  is  therefore  based  upon  the  posi- 
tion of  an  agent  of  metamorphism.  It  has  been  seen  that  the  zone  of  kata- 
morphism  is  separated  from  the  zone  of  anamorphism  by  a  reversal  of  the 
physical-chemical  factors.  As  one  would  suppose,  the  latter  distinction  is 
of  much  more  fundamental  importance  than  the  former. 

BELT  OF  WEATHERING. 

By  some  it  has  been  proposed  to  call  the  belt  of  weathering  that  of 
demorphism;  and  to  call  the  alterations  of  all  rocks  below  this  belt 
metamorphism.  The  fact  that  the  alterations  in  the  belt  of  weathering  are 
very  different  from  the  belts  below  has  been  well  known  for  many  years. 
But  it  has  not  been  generally  recognized  that  the  belts  of  weathering  and 
cementation  are  delimited  by  the  level  of  ground  water.  This  is  doubtless 
due  to  the  fluctuations  of  that  level  and  to  a  considerable  transition  band 
between  the  two  belts  (see  pp.  423-429,  560-561);  but  in  many  places  the 
change  in  the  character  of  the  alterations  in  passing  from  the  belt  of 
weathering  to  the  belt  of  cementation  is  very  sudden,  and  at  such  places 
is  very  clearly  connected  with  the  level  of  ground  water. 

The  belt  of  weathering  is  therefore  denned  to  extend  from  the  surface 
to  the  level  of  ground  water.  In  this  belt  all  of  the  very  important  reac- 
tions characteristic  of  the  zone  of  katamorphism — viz,  oxidation,  carbona- 
tion,  hydration,  and  solution — are  at  their  maximum  activity;  but  on  the 
whole,  of  these  three  reactions  the  most  characteristic,  but  not  the  dom- 
inant one,  is  that  of  the  carbonation  of  the  silicates.  This  reaction  takes 
place  on  a  vast  scale,  producing  carbonates  from  the  silicates,  and  at  the 
same  time  setting  free  silica  or  colloidal  silicic  acid.  Hydration  is  the  most 
extensive  simple  reaction  in  the  belt  of  weathering.  Oxidation  is  also  very 
important.  As  will  be  seen,  this  reaction  is  very  general  in  this  belt, 
because  not  being  saturated  with  water  the  oxygen  of  the  atmosphere  very 
rapidly  makes  its  way  through  the  porous  rocks  and  continually  supplies 
oxygen  to  replace  that  element  used  in  the  process  of  oxidation.  The  total 
effect  of  these  chemical  reactions  is  decomposition.  While  hydration  and 
oxidation  are  usual  for  this  belt,  under  special  conditions  these  reactions  may 
be  reversed.  In  places  of  luxuriant  vegetation  and  very  high  humidity 
deoxidation  may 'take  place.  In  regions  of  great  heat  and  temporary  or 


164  A  TREATISE  ON  METAMORPHISM. 

permanent  aridity  dehydration  may  locally  occur.  As  already  noted,  as  a 
result  of  oxidation,  carbonation,  and  hydration,  the  volume  of  the  rocks 
would  be  greatly  increased  if  all  the  compounds  formed  remained  in  situ; 
but  the  complex  process  of  solution  is  dominant.  Many  of  the  compounds 
formed  are  dissolved  in  large  quantities  and  transferred  by  the  overground 
water  circulation  to  the  sea,  or  by  the  underground  water  circulation  to  the 
belt  of  cementation  below.  Consequently  the  volume  of  the  rocks  contin- 
uously decreases  in  the  belt  of  weathering;  and  finally  the  resultant  material 
may  occupy  but  a  small  fraction  of  the  original  volume. 

In  the  belt  of  weathering,  in  addition  to  the  characteristic  chemical 
reactions,  mechanical  disintegration  is  the  rule.  Thus  the  complex  results 
of  weathering  may  be  classified  into  disintegration,  decomposition,  and 
solution.  As  a  final  result  of  the  various  mechanical  and  chemical  changes, 
rocks  soften  and  degenerate.  As  coherent  solids  they  are  destroyed.  The 
processes  of  the  belt  of  weathering  are  therefore  destructive.  The  minerals 
which  remain  are  usually  few  and  simple,  and  ordinarily  are  not  well 
crystallized.  In  the  destructive  processes  all  of  the  agents  of  meta- 
morphism,  both  inorganic  and  organic,  are  actively  at  work.  The  details 
of  these  processes  are  fully  developed  in  Chapter  VI,  on  "  The  belt  of 
weathering." 

BELT  OF  CEMENTATION. 

The  belt  of  cementation  extends  from  the  bottom  of  the  belt  of 
weathering  to  the  bottom  of  the  .zone  of  katamorphism.  On  the  average 
this  belt  is  therefore  much  thicker  than  the  belt  of  weathering.  All  of  the 
very  important  reactions  characteristic  of  the  zone  to  which  the  belt 
belongs — viz,  oxidation,  carbonation,  and  hydration — take  place.  Water 
is  everywhere  abundantly  present  in  the  belt,  and  hence  hydration  is  the 
most  important  of  the  three  reactions.  The  minerals  produced  by  meta- 
somatic  change  from  the  original  minerals  and  those  deposited  from  the 
solutions  are  likely  to  be  strongly  hydrated.  The  processes  of  carbonation 
and  oxidation  in  the  belt  of  cementation  are  largely  limited  by  the  amount 
of  carbon  dioxide  and  oxygen  there  contained. 

It  will  be  seen  (pp.  608-610)  that  carbon  dioxide  is  derived  from  several 
sources  and  that  carbonation  is  usual  throughout  the  belt,  but  that  the 
oxygen  is  limited  to  that  derived  from  above,  and  consequently  that  oxidation 


REACTIONS  IN  BELT  OF  CEMENTATION.  165 

is  usual  iu  only  a  very  limited  part  of  the  belt.  Not  only  are  the  processes 
of  earbonation  and  oxidation  subordinate  to  hydration,  but  the  process  of 
oxidation  not  infrequently  is  stopped  or  reversed  in  all  but  the  upper  part 
of  the  belt  of  cementation.  This  anomaly  is  due  to  the  fact  that  many  of 
the  rocks  contain  organic  materials  or  sulphides  or  both  which  have  a 
strong  affinity  for  oxygen.  When  the  oxygen  is  exhausted  from  the  water 
derived  from  the  belt  of  weathering  the  reducing  compounds  may  act 
directly  as  reducing  agents  or  may  produce  reducing  solutions.  The 
demands  of  these  reducing  agents  for  oxygen  may  abstract  this  material 
from  highly  oxidized  compounds,  such  as  ferric  oxide,  basic  ferric  sulphate, 
etc.  Deoxidation  in  the  belt  of  cementation  is  most  commonly  the  result  of 
the  burial  of  the  higher  oxide  of  iron  and  sulphates  with  a  considerable 
amount  of  organic  material  in  the  presence  of  abundant  water.  Under 
these  circumstances  the  ferric  compounds  may  be  reduced  to  ferrous 
compounds  and  the  sulphates  to  sulphides. 

But  it  is  to  be  noted  that  the  reduction  of  these  compounds  involves 
simultaneous  oxidation  of  the  organic  compounds,  the  resultant  products 
being  C02  and  water.  The  carbon  dioxide  may  escape  from  the  belt  or 
enter  iiito  other  combinations.  For  instance,  as  explained  fully  in  another 
place,  the  ferrous  compounds  largely  unite  with  the  carbon  dioxide,  pro- 
ducing carbonates.  Similar  reactions  may  take  place  with  reference  to 
other  less  abundant  metals,  as,  for  instance,  manganese,  and  some  metals 
may  even  be  reduced  to  the  metallic  condition,  for  instance,  copper,  silver, 
and  gold.  These  reducing  reactions  in  the  belt  of  cementation,  except  in 
the  case  of  iron,  are  of  small  consequence  from  a  geological  point  of  view, 
but  they  have  a  most  important  bearing  upon  the  deposition  of  ores.  (See 
Chapter  XII.)  It  thus  appears  that  oxidation  and  deoxidation  are  both 
rather  important  in  the  belt  of  cementation. 

The  changes  in  the  belt  of  cementation  ordinarily  produce  crystalline 
minerals  Minerals  which  were  partly  altered  by  processes  in  the  belt  of 
weathering  may  be  regenerated.  This  applies  only  to  those  minerals  which 
are  adapted  to  the  belt  of  cementation.  The  average  specific  gravity  of  the 
rocks  is  usually  lessened. 

It  has  been  noted  that  the  most  characteristic  reaction  of  the  belt 
of  weathering  is  solution.  In  contrast  with  this  the  most  characteristic 
reaction  of  the  belt  of  cementation  is  deposition  in  the  openings  of  the 


166  A  TREATISE  ON  METAMORPHISM. 

rocks.  The  material  deposited  is  derived  from  the  belt  of  weathering  or 
from  the  alterations  within  the  belt  of  cementation  itself.  Much  of  the 
material  dissolved  in  the  belt  of  weathering  is  continuously  transferred  to 
the  belt  of  cementation  by  the  downward  movement  of  water.  The  total 
amount  of  material  which  is  thus  derived  from  the  belt  of  weathering  is  not 
limited  to  the  thin  belt  which  exists  at  any  given  time;  for,  as  a  result  of 
denudation,  the  belt  of  weathering  is  constantly  migrating  downward  and 
encroaching  upon  the  upper  part  of  the  belt  of  cementation;  and  thus 
there  is  never  a  lack  of  material  for  solution  in  the  belt  of  weathering  which 
may  be  dissolved  and  transferred  to  the  belt  of  cementation.  Within 
the  belt  of  cementation  itself  the  reactions  of  oxidation,  carbonation,  and 
hvdratiou  all  increase  the  volume,  provided  all  the  compounds  formed,  or 
a  large  part  of  them,  remain  as  solids.  The  material  added  to  the  belt  of 
cementation  from  the  belt  of  weathering,  and  the  reactions  within  the  belt 
of  cementation,  furnish  an  abundant  supply  of  material  for  deposition  in 
the  openings  of  the  rocks,  whether  these  openings  be  those  originally 
present  or  produced  by  erogenic  forces.  And,  as  a  matter  of  fact,  in  the 
belt  of  cementation  the  openings  are  continuously  filled  by  mineral 
matter  and  finally  closed;  but  this  does  not  show  that  solution  may  not 
preponderate  over  deposition  in  this  belt  if  the  effect  upon  the  original  rocks 
and  the  openings  both  be  considered.  (See  pp.  612-617.)  The  mechanical 
result  of  the  various  processes  is  to  indurate  the  rocks.  The  processes  of 
the  belt  of  cementation  are  constructive.  The  belt  of  cementation,  from  a 
geological  point  of  view,  is  fully  considered  in  Chapter  VII. 

BELTS   OF  WEATHERING  AND   CEMENTATION   CONTRASTED. 

The  alterations  in  the  belts  of  weathering  and  cementation,  while  not 
so  fundamentally  different  as  those  in  'the  zones  of  katamorphism  and 
anamorphism,  contrast  strongly.  In  the  belt  of  weathering,  of  the  great 
reactions  characteristic  of  the  zone  of  katamorphism — oxidation,  carbonation, 
and  hydration — all  are  important,  but  carbonation  is  most  characteristic.  In 
the  belt  of  cementation,  of  these  reactions  hydration  is  most  important.  In 
the  belt  of  weathering,  solution  greatly  dominates  over  deposition.  In  the 
belt  of  cementation  solution  and  deposition  are  more  nearly  balanced,  but 
because  of  reactions  which  increase  the  volume  of  the  rocks  the  openings  are 


REACTIONS  OF  ZONE  OF  ANAMORPHISM.  167 

filled.  In  the  belt  of  weathering,  the  material  continuously  decreases  in 
volume  due  to  solution;  in  the  belt  of  cementation  it  continually  increases  in 
volume  due  to  deposition  of  material  through  reactions  involving  expansion 
of  volume.  These  changes  of  volume  due  to  addition  or  subtraction  of 
material  commonly  involve  decrease  in  specific  gravity.  In  the  belt  of 
weathering  the  mechanical  results  are  disintegration  and  softening;  in  the 
belt  of  cementation,  cementation  and  induration.  The  belt  of  weathering 
is  therefore  especially  characterized  by  solution,  decrease  of  volume,  and 
softening,  resulting  in  physical  degeneration.  The  belt  of  cementation  is 
especially  characterized  by  deposition,  increase  of  volume,  and  induration, 
resulting  in  physical  coherence. 

ZONE  OF  ANAMORPHISM. 

At  a  variable  depth  below  the  surface  of  the  earth  the  pressure  is  so 
great  that  it  can  not  be  supposed  that  considerable  openings  permanently 
exist.  The  depth  at  which  this  condition  of  affairs  is  reached  depends 
largely  upon  the  character  of  the  rocks.  For  the  strong  rocks,  as  already 
noted  (p.  160),  this  depth,,  under  quiescent  geological  conditions,  may  be  as 
great  as  10,000  or  12,000  meters.  If  openings  be  originally  present  in 
the  rocks  of  the  zone  of  anamorphism,  as,  for  instance,  sandstones,  vesicular 
lavas,  etc.,  or  be  due  to  fracture  while  the  rocks  are  not  deeply  buried, 
when  such  rocks  become  sufficiently  deeply  buried  to  be  in  the  zone  of 
anamorphism,  it  is  certain  that  rock  flowage  will  take  place  and  the 
openings  will  be  closed,  except  possibly  those  of  subcapillary  size  and 
other  minute  openings  in  which  water,  carbon  dioxide,  or  other  liquids  and 
gases  are  occluded.  In  the  zone  of  anamorphism  there  is  great  pressure  in 
all  directions,  and  mechanical  energy  becomes  the  dominant  factor  which 
controls  the  reactions.  Changes  consequently  take  place  which  diminish 
the  volume  of  the  rocks.  This  volume  change  increases  the  specific  grav- 
ity, and  contrasts  with  the  volume  changes  of  the  zone  of  katamorphism. 
The  fundamental  chemical  law  of  energy  in  reference  to  heat  is  subordinate. 
Reactions  take  place  with  the  liberation  or  absorption  of  heat,  depending 
upon  what  is  demanded  by  the  pressure.  Commonly,  the  preponderant 
chemical  reactions  are  those  which  take  place  with  absorption  of  heat 
The  depth  at  which  pressure  becomes  dominant  is  variable,  depending 


168  A  TREATISE  ON  METAMORPHISM. 

upon  the  character  of  the  rock  and  upon  whether  the  conditions  are  mass- 
static  or  mass-mechanical. 

It  has  been  seen  that  at  the  moderate  temperatures  of  the  zone  of  kata- 
morphism  the  preponderant  chemical  reactions  are  those  which  take  place 
with  the  liberation  of  heat.  As  the  depth  below  the  surface  increases,  the 
temperature  ever  becomes  higher;  and  consequently  the  temperature  may 
become  so  high  that  the  tendency  for  chemical  reactions  to  take  place 
with  the  liberation  of  heat  is  less  dominant,  and  at  sufficiently  great  depths 
the  heat  may  be  so  great  that  this  tendency  ceases,  or  is  even  reversed. 
Or,  using  the  words  of  vaii't  Hoff,  at  high  temperatures  the  preponderating 
chemical  reactions,  or  associations,  which  take  place  at  lower  temperatures 
with  the  development  of  heat  are  replaced  by  preponderating  chemical 
reactions^  or  dissociations,  which  take  place  with  the  absorption  of  heat." 
However,  at  moderate  depths  in  the  zone  of  anamorphism  under  ordinary 
conditions  the  temperatures  are  not  very  high.  For  instance,  at  a  depth  of 
9,000  meters  the  temperature  is  probably  in  the  neighborhood  of  300°  C. 
Therefore,  so  far  as  the  temperature  is  concerned,  for  that  part  of  the  crust 
of  the  earth  within  observation  the  preponderant  chemical  reactions  would 
probably  take  place  under  the  first  part  of  van't  Hoff 's  law,  rather  than  under 
the  second  part,  if  it  were  not  for  the  pressure.  But  the  pressure  is  the 
dominant  factor  which  controls  the  reactions.  The  rocks  in  this  zone  are 
under  so  great  pressure  in  all  directions  that  this  fact  demands  chemical 
reactions  which  produce  diminished  volumes  irrespective  of  whether  heat  is 
liberated  or  absorbed  by  them. 

The  very  important  reactions  in  the  zone  of  anamorphism  are  silica- 
tion,  or  union  of  silicic  acids  with  bases  producing  silicates,  and  dehydra- 
tion. Deoxidation  is  subordinate  The  process  of  silication  commonly 
takes  place  upon  carbonates,  and  consequently  involves  decarbonation  and 
the  liberation  of  the  carbon  dioxide,  which  may  escape  and  thus  the  volume 
be  decreased.  To  what  extent  the  pressure  is  the  controlling  factor  in  the 
production  of  this  reaction  is  difficult  to  say.  Probably  it  is  the  dominant 
cause,  but  it  is  possible  that  at  the  temperatures  which  prevail  in  this  zone 
silicic  acid  may  be  relatively  more  active  than  at  the  lower  temperatures  of 
the  zone  of  katamorphism,  where  carbonic  is  the  stronger  acid. 

"NernBt,  W.,  Theoretical  chemistry,  translated  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
p.  583. 


REACTIONS  OF  ZONE  OF  ANAMORPHISM.  169 

As  illustrations  of  the  process  of  silication  may  be  mentioned  the 
formation  of  wollastonite  from  pure  limestone,  of  tremolite  from  dolomitic 
limestone,  of  actinolite  from  ankerite,  and  of  grunerite  from  siderite.  (See 
pp.  239,  241,  243,  244.)  In  the  impure  limestones  under  deep-seated  con- 
ditions, where  numerous  bases  are  present,  various  complicated  silicates 
form,  such  as  other  pyroxenes  and  amphiboles,  tourmaline,  choudrodite,  etc. 

The  process  of  dehydration  involves  the  liberation  of  water.  This 
reaction,  it  is  safe  to  say,  is  one  which  is  controlled  by  pressure.  The 
combined  water  is  actually  squeezed  out  of  the  hydrated  mineral  particles, 
transforming  them  to  less  hydrous  and  to  anhydrous  forms  in  a  manner 
similar  to  that  in  which  free  water  is  pressed  from  a  sponge. 

Whether  or  not  pressure  in  the  zone  of  anamorphism  is  sufficient  to 
deoxidize  compounds  is  uncertain.  Certainly  it  can  not  be  asserted  that 
the  pressure  is  sufficient  to  squeeze  out  a  part  of  the  oxygen  of  hematite, 
thus  transforming  it  to  magnetite.  So  far  as  deoxidation  occurs,  probably 
the  oxygen  abstracted  from  the  rocks  usually  unites  with  the  elements  of 
organic  compounds,  thus  producing  carbon  dioxide  and  water.  Thus  the 
chief  products  liberated  by  silication,  dehydration,  and  deoxidation  are  car- 
bon dioxide  and  water.  These  join  the  interstitial  water  in  the  subcapillary 
spaces  and  probably  slowly  escape  into  the  zone  of  katamorphism  above. 
(See  pp.  665—667.)  This  results  in  loss  of  material,  and  since  the  specific 
gravity  of  the  minerals  is  increased  on  the  average,  the  volume  of  the 
rocks  is  decreased. 

Besides  the  above  processes,  condensation  may  also  be  accomplished 
by  recrystallization,  although  this  process  generally  takes  place  in  connec- 
tion with  them.  The  process  of  recrystallization  produces  a  rearrangement 
of  the  elements  in  such  a  way  as  to  form  compounds  of  higher  specific 
gravity.  This  is  well  illustrated  by  the  devitrification  of  glass. 

The  minerals  produced  in  the  zone  of  anamorphism  are  numerous, 
definite,  stable,  crystalline,  of  high  specific  gravities,  and  probably  have 
complex  molecules.  The  rocks  formed  are  compact  and  strong.  The 
lower  zone  may  therefore  properly  be  called  the  zone  of  anamorphism,  or 
anamorphic  zone.  This  use  of  the  term  anamorphism  is  parallel  to  the  use 
of  the  term  anabolism  in  biblogy  to  designate  those  chemical  changes  in  a 
living  body  which  result  in  the  production  of  complex  compounds  from  more 


170  A  TREATISE  ON  METAMORPHISM. 

simple  ones.  The  zone  of  anamorphism  may  be  defined  as  the  zone  in 
which  alterations  of  rocks  result  in  the  production  of  complex  compounds 
from  more  simple  ones. 

Summarizing  the  energy  factors  in  the  zone  of  anamorphism,  so  far 
as  the  volume  change  is  concerned,  the  result  is  to  liberate  heat;  so  far 
as  the  chemical  reactions  are  concerned,  heat  may  be  liberated  or  absoi'bed, 
but  the  latter  reaction  is  more  common.  In  the  latter  case  the  heat 
absorbed  is  almost  certainly  much  greater  than  that  liberated  by  decrease  of 
volume.  If  it  were  not  that  a  considerable  number  of  the  chemical  reactions 
liberate  heat,  it  would  be  certain  that  heat  is  absorbed  in  the  zone  of 
anamorphism  But  the  heat  liberated  by  some  chemical  reactions  must 
be  added  to  that  liberated  by  decrease  of  volume.  Whether  this  sum  is  as 
great  as  the  heat  absorbed  by  the  preponderating  chemical  reactions  is 
somewhat  uncertain;  but  it  is  thought  to  be  rather  probable,  for  the  com- 
pounds immediately  concerned  in  the  reactions,  that  the  total  effect  is  to 
absorb  heat  and  store  energy.  However,  in  order  to  accomplish  this,  energy 
must  be  derived  from  an  outside  source,  and  when  all  the  factors  which  in 
any  way  affect  the  reactions  are  taken  into  account,  including  the  movement 
of  the  superincumbent  material,  heat  is  dissipated  and  energy  lost.  (See 
p.  182.) 

RELATIONS  OF  ZONES  OF  KATAMOKPHISM  ATSTD  ANAMORPHISM. 

We  shall  now  consider  the  zones  of  metamorphism  together  with 
reference  to  the  energy  factors.  So  far  as  the  chemical  reactions  are 
concerned,  it  has  been  seen  that  they  may  take  place  with  liberation  or 
absorption  of  heat.  So  far  as  heat  is  liberated  energy  is  dissipated.  So 
far  as  heat  is  absorbed  energy  is  stored.  The  change  in  volume  may  also 
result  in  the  dissipation  or  storage  of  energy.  Where  increase  of  volume 
is  preponderant  energy  may  be  stored  (1)  by  increasing  the  volume  of  the 
rocks  affected  by  the  reaction  or  (2)  by  elevating  the  overlying  rocks  in 
order  that  the  space  shall  be  available  for  the  expenditure.  In  a  given 
case  the  energy  may  be  stored  by  (1)  or  (2)  or  a  combination  of  them. 
Where  decrease  of  volume  is  preponderant  energy  is  dissipated  (1)  by  the 
decrease  of  volume  of  the  rock  affected  by  the  reaction  or  (2)  by  subsid- 
ence of  the  overlying  material,  or  by  both.  Below  the  extreme  outer  film 
of  the  earth  the  factor  of  elevation  or  subsidence  of  the  overlying  rocks  is 
of  vastly  greater  importance  than  the  volume  change,  and  the  relative 


RELATIONS  OF  ZONES.  171 

importance  of  this  factor  steadily  increases  with  depth.  This  is  more 
broadly  true  in  the  case  of  increase  of  volume  than  in  that  of  decrease 
of  volume;  for  in  the  latter  case  in  the  zone  of  katamorphism  the  strength 
of  the  rocks  near  the  surface  may  prevent  subsidence,  and  the  decrease  of 
volume  simply  produce  porosity.  A  common  illustration  of  that  is  vesicu- 
lar dolomite.  However,  in  the  zone  of  anamorphism,  when  the  reactions 
result  in  decrease  of  volume,  subsidence  occurs  and  energy  is  dissipated. 
The  importance  of  the  necessity  of  lifting  the  overlying  material  in  order 
to  find  more  room  in  the  case  of  increase  of  volume  is  well  illustrated  by 
the  frequent  rapid  hydration  or  slacking,  with  great  expansion  and  rapid 
disintegration,  which  follows  when  a  partly  hydrated  rock,  buried  but  a 
few  feet,  is  brought  to  the  surface."  Apparently  when  buried  the  tendency 
for  hydration  and  necessary  expansion  with  liberation  of  heat  was  not 
sufficient  to  lift  the  superjacent  material.  When  the  necessity  of  elevat- 
ing the  superjacent  material  was  removed  by  transfer  to  the  surface  the 
process  of  hydration  and  expansion  went  on  to  completion  with  great 
rapidity. 

I  conclude  from  the  foregoing  that  in  so  far  as  energy  is  concerned 
there  are  four  cases.  Chemical  reaction  may  (1)  release  energy  and  result 
in  the  liberation  of  heat,  or  (2)  may  consume  energy  and  result  in  the 
absorption  of  heat.  The  change  of  volume  may  be  (3)  by  decrease  of 
volume,  and  result  in  the  release  of  energy  and  the  liberation  of  heat,  or 
(4)  by  increase  of  volume,  and  result  in  the  consumption  of  energy  and 
in  the  absorption  of  heat.  (1)  and  (3)  will  be  called  plus,  and  when  they 
are  combined  the  heat  developed  is  equal  to  their  sum;  (2)  and  (4)  will 
be  called  minus,  and  when  they  are  combined  the  heat  absorbed  is  equal 
to  their  sum.  When  (1)  and  (4)  or  (2)  and  (3)  are  combined  heat  may 
be  liberated  or  absorbed,  and  consequently  energy  dissipated  or  stored, 
depending  upon  the  relative  values  of  the  opposing  factors. 

It  has  been  noted  that  the  three  important  reactions  in  the  zone  of 
katamorphism  are  oxidation,  carbonation,  and  hydration;  and  in  the  zone 
of  anamorphism  are  deoxidation,  silication,  and  dehydration. 

Since  all  of  the  abundant  metallic  elements  except  iron  are  completely 
oxidized  as  they  occur  in  the  original  rocks,  the  important  inorganic  com- 
pounds which  are  oxidized  in  the  zone  of  katamorphism  are  mainly  those 

"Merrill,  G.  P.,  Disintegration  of  the  granitic  rocks  of  the  District  of  Columbia:  Bull.  Geol.  Soc. 
America,  vol.  6,  1895,  p.  332. 


172  A  TREATISE  ON  METAMORPHISM. 

of  iron.  Iron  occurs  extensively  in  the  ferrous  form,  in  magnetite,  in  car- 
bonates, and  in  silicates.  To  a  considerable  extent  it  occurs  as  a  sulphide. 
To  a  small  extent  it  occurs  as  metallic  iron.  In  all  of  these  forms  it  is 
capable  of  oxidation.  The  main  result  of  the  oxidation  of  these  com- 
pounds, so  far  as  the  iron  is  concerned,  is  to  change  the  monoxide  to 
ferric  oxide.  But  where  it  is  present  as  a  sulphide  it  may  be  changed  to  a 
sulphate,  and  then  be  thrown  down  as  a  basic  ferric  sulphate.  Ferric  oxide, 
hydrous  or  anhydrous,  is  an  important  constituent  in  the  sedimentary 
rocks,  and  its  presence  is,  without  doubt,  largely  due  to  oxidation  in 
the  zone  of  katamorphism.  To  a  far  less  extent  other  metals,  such  as 
copper,  lead,  zinc,  etc.,  occur  in  the  native  form,  in  partially  oxidized 
forms,  or  as  sulphides.  All  these  substances  may  be  oxidized.  These 
substances  have  little  importance  in  general  geology,  but  are  of  great 
importance  in  the  production  of  ores.  All  of  the  reactions  of  oxidation 
take  place  with  great  liberation  of  heat  and  with  increase  of  volume.  In 
the  zone  of  anamorphism  partial  or  complete  deoxidation  of  the  highly 
oxidized  compounds  may  occur.  The  ferric  iron  may  be  reduced  to  the 
ferrous  form.  The  sulphates  of  iron  and  the  other  metals  may  be  reduced 
to  sulphides.  In  most  cases  the  reducing  agent  is  organic  matter.  The 
reduction  of  the  metals  by  organic  compounds  results  in  the  oxidation  of 
the  carbon  and  hydrogen,  thus  producing  carbon  dioxide  and  water.  The 
carbon  dioxide  and  water  largely  escape.  Where  reducing  agents,  are  not 
present  the  highly  oxidized  materials  produced  in  the  zone  of  katamorphism 
commonly  remain  in  this  condition  even  if  the  material  passes  into  the 
zone  of  anamorphism.  Deoxidation  can  not,  therefore,  be  said  to  be  char- 
acteristic of  the  zone  of  anamorphism  to  the  degree  that  oxidation  is 
characteristic  of  the  zone  of  katamorphism.  The  reducing  reactions  all 
take  place  with  great  absorption  of  heat,  so  far  as  the  metals  are  concerned, 
and  with  decrease  of  volume.  However,  since  heat  is  liberated  by  the 
oxidation  of  the  carbon  and  hydrogen,  it  is  probable  that  the  sum  total 
of  the  heat  reaction  in  deoxidation  in  the  zone  of  anamorphism  is  to 
liberate  heat. 

In  the  matter  of  oxidation  and  deoxidation,  the  zone  of  katamorphism 
presents  a  case  in  which  the  chemical  law  of  the  liberation  of  heat  controls, 
without  reference  to  change  in  volume,  while  in  the  zone  of  anamorphism 
the  pressure  tending  to  produce  decrease  of  volume  and  chemical  reactions 
with  the  liberation  of  heat  probably  work  together. 


CHEMICAL  RELATIONS  OF  SILICON  AND  CARBON.  173 

Another  set  of  reactions,  of  the  most  fundamental  importance  and 
widespread  character,  which  occur  in  an  opposite  sense  in  the  two  zones  of 
metamorphism  are  the  mutual  replacements  of  carbon  dioxide  and  silicon 
dioxide.  It  has  already  been  noted  that  near  the  surface,  or  in  the  zone 
of  katamorphism,  carbonic  replaces  silicic  acid.  Deep  below  the  surface, 
or  in  the  zone  of  anamorphism,  silicic  replaces  carbonic  acid.  Under  the 
conditions  near  the  surface,  where  the  pressure  is  small  and  the  tempera- 
ture is  low,  carbonic  is  the  stronger  acid;  and  under  the  conditions  deep 
below  the  surface,  where  the  pressure  is  great  and  the  temperature  is 
high,  silicic  is  the  stronger  acid.  The  importance  of  the  mutual  replace- 
ment of  these  compounds  under  different  conditions  makes  it  advisable  to 
summarize  the  chemical  analogies  of  silicon  and  carbon.  Silicon  is  the 
characteristic  element  of  inorganic  compounds;  carbon  is  the  characteristic 
element  of  organic  compounds.  How  closely  analogous  are  these  two 
elements  is  shown  by  the  following  comparative  table: 

Chemical  relations  of  silicon  and  carbon. 


SiO2  silica,  anhydride,  solid  ......................  C02  carbon  dioxide,  gas. 

SiH4  silicon  hydride,  gas  .......  ..................  CH4  methane,  gas. 

SiCl4  silicon  chloride,  liquid  ......................  CC14  carbon  tetrachloride,  liquid. 

Boils  at  57°  ...............................  Boils  at  76°. 

SiHClj  silicon  chloroform,  liquid  .................  OHC1S  chloroform,  liquid. 

Boils  at  34°  ...............................  Boils  at  60°. 

Si  (C2H5)4  silicon  ethyl,  liquid  ..........  •  .........  C(C2H5)  4  tetraethylmethane,  liquid. 

Boils  at  150°."  ............................  Boils  at  120°. 

Si(OC2H5)4  ethyl  orthosilicate,  liquid.  -  ..  ..........  C(OC2H5)4  ethyl  orthocarbonate,  liquid. 

Boils  at  160°  ..............................  Boils  at  158°.      (See    Mendeleeff,  Vol.   II, 

Chap.  XVIII,  pp.  99-100.) 
H4SiO4  orthosilicic  acid  ..........................  H4CO4  orthocarbonic  acid. 

OH  OH 


\OH 
OH  OH 

SiO4(C2H5)4  ethyl  orthosilicate.  x 

(MgFe)2SiO4  olivine  Exists  only  in  certain  artificial  organic  compounds, 

CaAl2(SiO4)2anorthite          [Natural  orthosilicates.       as  ethyl  orthocarbonate,  CO4(C2H5)4. 

K"3R'"2  (SiO4)  3  garnet,  etc.) 

H2SiO3  metasilicic  acid    .........................  H2CO3  carbonic  acid. 

.OH  /OH 

0-Si^  0=C( 

XOH  XOH 

Exists   in   salts  and   in  solution.     Forms  normal 
(neutral)  and  acid  salts  ("bicarbonates"). 


174  A  TREATISE  ON  METAMORPHISM. 

Chemical  relations  of  silicon  and  carbon — Continued. 


SILICON. 


KAl(SiOs)2  leucite  (normal  salts) 


AlfO-Si=O 


\ 


O. 


K-0' 


o=c 


0-Na 


Natural  metasilicates.  O— Na 

O=C\    /Ca 

etc.  (acid  salts) 

/OH 

o=c( 

xONa 
or 

/OH 
0=CC 

°N_ 


0=C 


O' 


"OH 

H,SijO5  disilicic  or  dimetasilicicacid.....  .........  H2C2O5  dicarbonic  or  pyrocarbonic  acid. 

/OH  ,OH 

0=Si/  O=C^ 

>  )0 

0=Si(  O=C( 

^OH  NOH 

Si  :0  ::2  :7  for  basic  salts.  Known  only  in  salts,  as  C2O3(NaO)2,  produced  by 

Si  :  O  :  :  2  :  5  for  acid  salts.  heating  the  acid  salt. 

Not  known  in  free  state. 
LiAl  (Si2O5)j  petalite. 
H6Si2O,  diorthosilicic  acid  .......................  The  corresponding  carbon  acid  does  not  exist. 


\OH), 

H4Mg3Si2O9  serpentine  (normal  salt). 
0-Mg-OH 


/0>Mg 
Si-0X 


0-Mg-OH 
H2CaSi2O6+H.jO  okenite  (acid  salt). 
H4SisO8  polysilicic  acid  or  trisilicic  acid  ...........  The  corresponding  carbon  acid  does  not  exist. 

(May  be  considered  as  metasilicic  acid  plus 
disilicic  acid.  ) 
/OH 
Si=O 


8i=O 

>° 
Si=(OH), 

(neutral  salts  of  trisilicic  acid) 
KAlSi,O8  orthoclase. 
NaAlSi,O8  albite. 


VOLUMES  OF  SILICON  AND  CARBON  COMPOUNDS.  175 

Another  close  analogy  which  exists  between  the  carbonates  and  the 
silicates  is  the  fact  that  many  salts  of  both  give  alkaline  reactions,  or  under 
the  theory  of  dissociation  are  hydrolized  as  explained  (pp.  86-87),  and 
that  alkalinity  increases  with  the  temperature. 

The  specific  volumes  of  the  silicates  and  carbonates  also  have  very 
close  relations.  In  general  the  specific  volumes  (the  molecular  weights 
divided  by  the  specific  gravities)  of  the  silicon  compound  are  slightly  the 
greater.  The  comparative  specific  volumes  of  a  number  of  the  correlative 
silicon  and  carbon  compounds  are  as  follows:" 


Specific  volumes  of  silicon  and  carbon  compounds. 

CC14 94 

CHC1S 81 

C(OC2H5), 186 

CaCOj 37 


SiCl4 112 

SiHCL, 82 

Si(OC2H6)4 201 

CaSiO3  .  .     41 


The  specific  volumes  of  Si02  and  C02  are  wholly  different,  but  this  is 
explained  by  the  fact  that  one  is  a  solid  and  the  other  a  gas. 

Since  the  specific  volumes  of  the  carbon  compounds  are  less  than  those 
of  the  silicon  compounds,  if  there  be  a  simple  substitution  of  carbon  for 
silicon  the  volume  is  decreased;  if  silicon  for  carbon,  the  volume  is 
increased.  However,  as  a  matter  of  fact,  the  changes  in  the  rocks  are 
never  so  simple  as  this.  The  volume  changes  in  carbonation  with  desilica- 
tion,  and  in  silication  with  decarbonation  in  the  rocks  largely  depend  upon 
whether  the  reacting  and  resultant  compounds  are  gaseous,  liquid,  or  solid, 
and  whether  the  products  remain  as  solids  or  are  dissolved  and  transported 
elsewhere. 

In  the  zone  of  katamorphism  carbon  dioxide  replaces  silicon  dioxide 
ordinarily  with  liberation  of  heat 

The  fact  of  the  carbonation  of  the  silicates  is  well  known.  So  far  as  I 
know,  the  importance  of  this  process  was  first  realized  by  Bischof.  He 
attributes  the  general  decomposition  of  the  rocks  near  the  surface  mainly 
to  the  action  of  carbonic  acid,  thus  producing  the  carbonates  which  are 
found  in  spring  water.  He  shows  by  experiment  that  "the  silicates  of 
alkalies,  alkaline  earths,  protoxides  of  iron  and  manganese  are  decomposed 

"  Mendel£eff ,  D.,  The  principles  of  chemistry,  translated  by  Geo.  Kamensky,  Longmans,  Green 
&  Co.,  London,  189",  vol.  2,  pp.  99-100. 


176  A  TREATISE  ON  METAMORPHISM. 

by  carbonic  acid  at  ordinary  temperatures."*  But  he  says  that,  since 
carbonic  acid  does  not  combine  with  alumina  or  peroxide  of  iron,  the 
silicates  of  these  compounds  are  not  decomposed  by  carbonic  acid."  How- 
ever, we  now  know  that  the  process  of  carbonation  takes  place  with  all  the 
natural  silicates.  It  will  be  shown  in  Chapter  VII  that  this  process  of 
carbonation  goes  on  throughout  the  entire  zone  of  katamorphism,  but  it  is 
in  the  upper  of  the  two  belts  of  the  zone  of  katamorphism,  that  of  weath- 
ering, in  which  the  process  of  carbonation  goes  oil  with  greatest  rapidity 
and  is  especially  characteristic.  Simultaneously  with  the  substitution  of 
the  carbon  dioxide  for  the  silica  much  of  the  silica  separates  as  colloidal 
silicic  acid,  is  taken  into  solution,  and  is  carried  downward  to  the  belt  of 
cementation  by  the  percolating  waters.  In  this  belt  the  silica  is  deposited 
on  an  enormous  scale.  The  carbon  dioxide  is  furnished  in  solution,  being 
mainly  derived  directly  or  indirectly  from  the  atmosphere.  When  carbon 
dioxide  replaces  silicon  dioxide  the  volume  would  be  decreased,  provided 
all  of  the  silicic  acid  were  abstracted  in  solution.  But  it  is  probable  that 
the  larger  portion  of  the  silica  set  free  in  the  zone  of  katamorphism  by 
carbonation  is  deposited  in  the  belt  of  cementation,  and  therefore  the 
volume  of  the  zone  of  katamorphism  as  a  whole,  so  far  as  this  reaction  is 
concerned,  is  increased.  The  deposition  of  silica  in  the  belt  of  cementation 
is  probably  accompanied  by  a  considerable  absorption  of  heat,  under  the 
law  that  the  negative  value  of  the  heat  of  solution  is  greater  the  more 
insoluble  the  substance. 

Carbonation  in  the  zone  of  katamorphism  may  take  place  without 
replacing  silica,  as  in  the  case  of  the  union  of  carbon  dioxide  with  iron 
oxide  in  magnetite,  thus  producing  iron  carbonate.  In  this  case  the 
liberation  of  heat  and  the  increase  in  volume  are  both  great. 

In  the  zone  of  anamorphism,  and  especially  under  mass-mechanical 
conditions,  silica  replaces  carbon  dioxide  in  the  carbonates  on  the  most 
extensive  scale.  So  far  as  I  am  aware,  Bischof  was  the  first  to  realize  that 
under  proper  conditions  the  process  of  carbonation  of  the  silicates  could  be 
reversed.  He  shows  by  experiment  that  carbonates  of  calcium,  magnesium, 
and  iron  are  decomposed  by  silica  at  a  boiling  temperature,  and  cor- 

"  Bischof,  Gustav,  Elements  of  chemical  and  physical  geology,  translated  by  Paul  and  Drummond, 
Harrison  &  Sons,  London,  vol.  1,  1854,  p.  2. 
6  Bischof,  cit.,  vol.  1,  pp.  4-5. 


VOLUMES  OF  SILICON  AND  CARBON  COMPOUNDS.  177 

rectly  infers  that  when  any  of  these  carbonates  occur  with  quartz  at  a 
sufficient  depth  within  the  earth,  where  a  temperature  of  100°  C.  is  reached, 
this  reaction  may  take  place.  He  calculates  that  this  depth  will  be  2,440 
meters.  He  correctly  infers  that  the  presence  of  abundant  carbon  dioxide 
in  deep-seated  waters  is  probably  due  to  this  process  of  silication."  We 
now  understand  that  under  conditions  of  moderate  pressure  and  temperature 
not  only  are  the  carbonates  which  Bischof  mentioned  decomposed,  but  other 
carbonates  may  be  altered  in  a  similar  manner.  However,  it  is  noteworthy 
that  the  carbonates  which  Bischof  mentioned  are  those  of  predominant 
importance. 

The  substitution  of  silicon  for  carbon  would  result  in  increase  of 
volume  provided  silica  were  derived  from  the  solutions  and  the  carbon 
dioxide  passed  into  the  solutions.  But  in  the  process  of  silication  in  the 
belt  of  anamorphism  little  material  is  available  from  outside  sources. 
Therefore  the  most  of  the  silica  which  replaces  carbon  dioxide  in  carbonates 
must  be  considered  as  a  solid.  It  is  probable  that  a  large  part  of  the  freed 
carbon  dioxide  slowly  escapes;  for  at  temperatures  prevailing  in  the  zone 
of  anamorphism  the  carbon  dioxide  is  above  its  critical  temperature,  and 
therefore  a  gas,  and  probably  slowly  makes  its  way  through  the  subcapil- 
lary  spaces  to  the  zone  of  katamorphism  (see  p.  667.)  Hence  the  volume 
comparison  must  be  made  between  the  carbonate  and  replacing  silica 
combined  and  the  resultant  silicate.  On  this  basis  there  is  a  marked 
diminution  of  volume.  One  of  the  simplest  illustrations  of  the  formation 
of  the  silicates  with  condensation  of  volume  is  the  development  of  wollasto- 
nite  from  calcium  carbonate  and  quartz.  In  this  change  the  volume  of  the 
solid  remainder  is  decreased  31.48  per  cent.  However,  this  calculated 
decrease  is  somewhat  too  great;  for  it  will  be  seen  (p.  667)  that  some 
of  the  carbon  dioxide  does  not  escape,  but  is  retained  in  the  rocks  in  the 
form  of  numerous  inclusions. 

It  appears  from  the  foregoing  that  in  the  replacement  of  silicon  dioxide 
by  carbon  dioxide  in  the  zone  of  katamorphism,  the  chemical  law  of  reac- 
tions with  liberation  of  heat  dominates  over  that  of  pressure;  and  that  in 
the  substitution  of  silicon  dioxide  for  carbon  dioxide  in  the  zone  of 
anamorphism  the  physical  law  that  pressure  demands  decrease  of  volume 
dominates  over  the  chemical  law  of  reactions  with  liberation  of  heat. 

a  Bischof,  cit.,  vol.  1,  pp.  237-241. 
MON   XLVII — 04 12 


178  A  TREATISE  ON  METAMORPHISM. 

The  third  important  case  in  which  the  reactions  occur  in  the  opposite 
sense  in  the  zones  of  katamorphism  and  anamorphism  are  hydration  and 
dehydration. 

Hydration  is  a  characteristic  reaction  of  the  zone  of  katamorphism, 
only  less  important  than  that  of  carbonation ;  moreover,  hydratioii  occurs 
on  a  great  scale  both  in  the  belt  of  weathering  and  in  that  of  cementation. 
That  hydration  occurs  extensively  deep  in  the  belt  of  cementation  is 
evidenced  by  the  hydrated  minerals  which  develop  in  the  cavities  of  the 
rather  deeply  buried  rocks,  such  as  the  amygdules  of  amygdaloids. 
Hydration  represents,  in  the  words  of  the  first  part  of  van't  Hoff's  law,  "an 
association  which  takes  place  with  great  liberation  of  heat."  This  process 
also  results  in  very  considerable  increase  of  volume,  provided  all  or  nearly 
all  of  the  products  formed  remain  in  situ. 

Dehydration  is  a  characteristic  reaction  of  the  zone  of  anamorphism, 
only  less  important  than  that  of  silication.  When  the  hydrated  minerals 
formed  in  the  belt  of  katamorphism  pass  into  the  zone  of  anamorphism  by 
deep  burial  they  are  dehydrated.  The  pressure,  or  the  high  temperature, 
or  the  two  combined,  unite  to  drive  off  a  large  part  of  the  water.  Dehy- 
dration, in  the  words  of  the  second  part  of  van't  Hoff's  law,  represents  "  a 
dissociation  which  takes  place  with  great  absorption  of  heat"  and  it  takes 
place  with  decrease  of  volume. 

Therefore,  so  far  as  hydration  and  dehydration  are  concerned,  in  the 
upper  zone  the  first  part  of  van't  Hoff's  law,  that  of  chemical  reactions  with 
the  liberation  of  heat  obtains,  but  in  the  lower  zone  the  law  of  diminution 
of  volume  controls,  regardless  of  the  heat  effect.  The  first  part  of  this 
statement  is  sufficiently  evident;  the  second  possibly  needs  further  expla- 
nation. To  drive  off  the  combined  water  of  rocks  at  ordinary  pressure 
usually  requires  a  temperature  above  110°  C.  This  temperature  under 
mass-static  conditions  would  not  be  found  until  a  depth  of  3,300  meters  had 
been  reached.  It  is  certain  that  at  depths  much  less  than  this,  and  at 
temperatures  lower  than  this,  dehydration  takes  place  on  an  important  scale ; 
for  it  will  be  shown  (p.  744)  that  in  the  transformation  of  mudstones  to 
shales  there  is  a  loss  of  about  one-half  of  the  combined  water.  I  conclude 
that  under  many  circumstances  the  increase  in  temperature  is  not  suffi- 
cient to  reverse  the  reaction  of  hydration,  and  therefore  the  reversal  must 


VOLUME  EFFECTS  OE  HYDRATION  AND  DEHYDRATION.      179 

be  due  to  the  pressure.  However,  in  the  lower  part  of  the  zone  of  ana- 
morphism the  temperature  is  frequently  higher  than  110°  C.,  and  under 
such  circumstances  both  the  pressure  and  the  temperature  may  work  together 
to  produce  dehydration. 

The  statement  that  the  volume  is  decreased  by  dehydration  is  only 
true  provided  the  separated  water,  or  a  large  part  of  it,  escapes;  for  the 
volume  of  the  hydrated  solid  is  less  than  that  of  the  residual  solid  plus  the 
separated  water;  therefore,  if  the  water  could  not  escape,  pressure  would 
tend  to  preserve  the  combination.  Hence,  the  fact  that  the  reaction  does 
take  place  in  the  zone  of  anamorphism  shows  that  there  is  sufficient 
pressure  not  only  to  separate  the  combined  water  from  the  rocks,  making 
it  free  water,  but  to  squeeze  the  free  water  from  the  rocks  as  one  can 
squeeze  the  water  from  a  sponge.  The  effective  pressure  doing  the  work 
is  equal  to  the  pressure  of  the  adjacent  rocks  less  the  weight  of  an  equal 
column  of  water  extending  to  the  surface.  Thus,  under  mass-static  condi- 
tions, if  the  rocks  have  a  specific  gravity  of  2.7,  the  effective  weight  in 
producing  dehydration  and  driving  out  the  free  water  at  a  depth  of  3,300 
meters  is  that  of  a  column  of  material  of  this  height  with  specific  gravity 
of  1.7.  Under  mass-mechanical  conditions,  where  the  pressure  as  a  result 
of  thrust  may  be  much  greater  than  that  due  to  weight,  the  effective 
pressure  tending  to  separate  the  combined  water  is  much  greater.  Conse- 
quently, under  such  conditions  dehydration  may  occur  at  much  less  depth 
than  under  mass-static  conditions.  (See  pp.  766-768.) 

One  or  two  minerals  may  be  mentioned  which  illustrate  the  processes 
of  hydration  and  dehydration  in  the  two  physical-chemical  zones.  Near 
the  surface  and  to  a  considerable  depth,  under  mass-static  conditions, 
limonite  and  other  hydrated  oxides  of  iron  develop.  Deeper  down,  and 
especially  in  connection  with  mass-mechanical  action,  limonite  is  dehy- 
drated, and  hematite  is  produced.  As  another  illustration  may  be  men- 
tioned the  somewhat  similar  compounds,  chlorite  and  biotite.  Near  the 
surface  and  under  quiescent  geological  conditions  chlorite  forms.  Deep 
below  the  surface,  and  especially  under  mass-mechanical  conditions,  biotite 
ordinarily  develops.  This  is  nowhere  better  illustrated  than  in  the  Michi- 
gamme  formation  in  the  Marquette  district  of  the  Lake  Superior  region, 
where  these  two  minerals  directly  replace  each  other  under  the  law  just 


180  A  TREATISE  ON  METAMOKPHISM. 

stilted."  In  the  zone  of  katamorphism  the  complex  hydrous  silicates,  such 
as  the  kaolins,  serpentines,  and  zeolites  form.  In  the  zone  of  anamorphism 
these  minerals  are  largely  dehydrated,  and  such  minerals  as  muscovite, 
andalusite,  gajnet,  staurolite,  etc.,  are  produced. 

The  physical-chemical  principles  cited  (pp.  45-123)  give  reasons  for 
the  existence  of  the  above  reverse  sets  of  reactions  in  the  two  zones.  We 
can  now  give  chemical  or  physical  causes  why  oxidation,  carbonation,  and 
hvdratioii  take  place  in  the  zone  of  katamorphism,  and  deoxidation, 
silication,  and  dehydration  in  the  zone  of  anamorphism,  and  so  on  for  other 
reactions. 

While  each  of  these  sets  of  processes  is  particularly  characteristic  of 
one  zone,  it  is  not  meant  to  imply  that  each  reaction  may  not  occur  in 
both  zones.  But  in  the  zone  of  katamorphism,  oxidation,  carbonation, 
and  hydration  greatly  predominate  over  the  reverse  processes.  On  the 
other  hand,  in  the  zone  of  anamorphism,  deoxidation,  silication,  and  dehy- 
dration predominate  over  the  reverse  processes. 

If  all  of  these  sets  of  processes  reversed  as  preponderant  reactions  at 
the  same  depth,  it  would  be  possible  to  sharply  separate  the  zones  of 
katamorphism  and  anamorphism.  If,  for  instance,  for  a  given  region  above 
a  depth  of  10,000  meters  the  sum  totals  of  the  oxidation,  carbonation,  and 
hydration  were  greater  than  the  sum  totals  of  reverse  processes,  the  zone 
of  katamorphism  would  be  sharply  separated  from  the  zone  of  anamor- 
phism at  this  depth.  But  this  is  not  the  case.  The  reversal  of  each  pair 
of  processes  occurs  at  different  depths;  and,  further,  the  reversal  for  a 
given  pair  of  processes  is  at  different  depths  under  different  conditions. 
One  of  the  most  important  of  these  is  as  to  whether  the  conditions  are 
mass-static  or  mass-mechanical. 

Of  the  tliree  sets  of  reversing  reactions,  oxidation  and  deoxidation, 
carbonation  and  silication,  hydration  and  dehydration,  the  first  reverses 
with  the  least  depth  and  pressure,  the  second  requires  the  greatest  depth 
and  pressure,  and  the  last  a  mean  depth  and  pressure.  It  has  already  been 
noted  that  oxidation  very  frequently  is  replaced  by  deoxidation  in  the 
lower  part  of  the  zone  of  katamorphism.  It  is  certain  that  the  process  of 
hydration  is  very  greatly  stayed,  if  it  does  not  altogether  cease,  and  may 

«  Van  Hise,  C.  R.,  and  Bayley,  W.  8.,  The  Marquette  iron-bearing  district  of  Michigan:  Mon.  U.  S. 
Geol.  Survey,  vol.  28,  1897,  pp.  444-459. 


CONTRASTING  REACTIONS  OF  DIFFERENT  ZONES.  181 

even  be  reversed  in  the  lower  part  of  the  zone  of  katamorphism.  It  is 
therefore  apparent  that  the  two  zones  are  not  sharply  delimited.  In 
general,  however,  it  may  be  said  that  the  outer  zone  to  a  depth  in  which 
oxidation,  carbonation,  and  hydration  preponderate  is  that  of  katamor- 
phism,  and  that  the  deeper-lying-  zone,  in  which  the  reverse  of  these 
processes  preponderate,  is  that  of  anamorphism.  But  carbonation  and  its 
opposite,  desilication,  are  the  most  fundamental  reactions  of  the  zone  of 
katamorphism.  Silication  and  decarbonation  are  the  most  fundamental 
reactions  of  the  zone  of  anamorphism.  By  these  reactions  more  than  by 
any  others,  these  zones  are  delimited.  The  three  sets  of  reversing  reac- 
tions, oxidation  and  deoxidation,  carbonation  and  silication,  hydration  and 
dehydration,  constitute  three  cycles  in  metamorphism.  The  second  of 
these  cycles  was  recognized  many  years  ago  by  Bischof  (see  pp.  176-177), 
and  was  called  the  carbono-silicic  cycle. 

From  the  foregoing  statement  it  is  clear  that  the  work  of  the  zones  of 
katamorphism  and  anamorphism  are  opposed  to  each  other.  What  the  one 
is  doing  the  other  is  undoing.  At  the  present  time  it  is  therefore  possible 
that  in  the  case  of  any  one  of  the  pairs  of  opposed  reactions,  consider- 
ing both  the  zones,  either  one  of  them  preponderates,  or  that  they  are 
approximately  balanced.  For  instance,  the  amount  of  water  being  fixed 
in  the  zone  of  katamorphism  may  be  greater  or  less  than  the  amount  of 
water  being  freed  by  dehydration  in  the  zone  of  anamorphism,  or  the 
t\vo  may  be  nearly  balanced.  The  same  statement  may  be  made  in  refer- 
ence to  the  other  reversing  reactions.  Upon  the  preponderance  of  these 
opposing  sets  of  reactions  in  the  opposite  zones  depends  the  answer  to 
the  question  whether,  on  the  whole,  oxygen,  carbon  dioxide,  and  water 
from  the  atmosphere  and  hydrosphere  are  being  .fixed  or  freed  by  meta- 
morphism. This  question  is  considered  in  Chapter  XI. 

\Vliile  the  zones  of  katamorphism  and  anamorphism  are  separated 
from  each  other  by  contrasting  reactions,  all  reactions  do  not  reverse  in  the 
two  physical-chemical  zones.  The  first  part  of  van't  Hoff's  law  of  heat 
and  the  law  of  pressure  may  work  together — that  is,  in  both  zones  reactions 
may  occur  which,  simultaneously  with  the  liberation  of  heat  by  chemical 
action,  also  result  in  liberation  of  heat  by  condensation.  In  so  far  as 
there  are  cases  of  this  kind  it  is  to  be  presumed  that  such  reactions  are 
common  to  both  zones.  As  an  instance  in  which  heat  is  probably  evolved 


182  A  TREATISE  ON  METAMORPHISM. 

both  by  the  chemical  reactions  and  by  the  volume  change  in  both  zones  may 
be  mentioned  the  devitrification  of  glass.  (See  Chapter  V,  pp.  251-252.) 
The  chemical  reaction  is  presumably  under  the  first  part  of  van 't  Hoff's 
la-w,  and  the  volume  is  decreased.  Another  instance  of  chemical  reaction 
with  the  liberation  of  heat  and  condensation  of  volume  is  the  replacement 
of  calcium  by  magnesium  in  limestone,  thus  transforming  the  rock  into 
dolomite." 

It  is  thought  to  be  certain  that  the  total  of  all  the  changes  taking  place 
in  the  whole  of  the  mass  of  rocks  concerned  in  any  given  modification  of 
the  lithosphere  results  in  the  dissipation  of  energy,  and  it  is  believed  that 
such  is  the  fact  for  each  of  the  physical-chemical  /ones  separately.  In  the 
zone  of  katamorphism  the  chemical  reactions  result  in  liberation  of  heat; 
the  average  volume  reaction  results  in  absorption  of  heat.  It  is,  however, 
thought  certain  that  the  residual  is  in  favor  of  the  former.  In  the  zone 
of  anamorphism  the  average  of  the  chemical  reactions  results  in  absorption 
of  heat;  the  average  of  the  volume  reactions  results  in  the  liberation  of 
heat.  It  has  already  been  seen  (pp.  170-171)  that  the  amount  of  energy 
required  for  the  volume  change  rapidly  increases  with  depth,  and  in  the 
lower  zone  it  is  thought  that  the  heat  liberated  from  the  volume  changes  is 
greater  than  the  heat  absorbed  by  the  chemical  reactions,  and  therefore 
that  the  residual  is  in  favor  of  the  liberation  of  heat. 

Hence,  it  is  concluded  that  the  changes  which  take  place  in  each  of 
the  zones  are  under  the  general  law  of  the  running  down  of  energy  into 
the  form  of  heat  which  is  dissipated,  and  this  accords  with  the  apparent 
order  of  the  universe. 

A  corollary  to  the  foregoing  pages  is  the  conclusion  that  in  the  upper 
zone,  where  pressure  is  relatively  unimportant,  on  the  average,  alterations 
result  in  the  expansion  of  the  volume  of  the  rocks;  and  that  in  the 
deeper-seated  zone,  where  pressure  is  important  or  dominant,  on  the  average 
the  alterations  result  in  the  contraction  of  the  volume  of  the  rocks.  It 
follows  as  a  further  conclusion  from  this  that  the  tendency  of  the  alterations 

"The  verification  from  authorities  of  the  heat  of  the  chemical  reactions  and  the  volume  relations 
for  the  majority  of  the  changes  above  mentioned  have  been  very  kindly  made  for  me  by  Mr.  A.  T. 
Lincoln.  Mr.  Lincoln  either  has  found  the  results  used  in  the  works  of  Thomsen,  Ostwald,  Mendeleeff, 
or  other  standard  authorities,  or  from  the  data  there  found  has  been  able  to  calculate  results  which 
answer  the  specific  questions  I  gave  to  him. 


SPECIFIC  GRAVITIES  OF  MINERALS  IN  THE  ZONES.  183 

in  the  first  zone  is,  on  the  average,  to  produce  minerals  of  lower  specific 
gravity  than  the  original  minerals,  while  in  the  deeper-seated  zone  the 
tendency,  on  the  average,  is  to  produce  minerals  of  higher  specific  gravity. 

Illustrations  of  the  first  rule  are  the  minerals  produced  by  the  disinte- 
gration and  decomposition  of  rocks  near  the  surface,  out  of  which  the  sedi- 
mentary rocks  are  built.  Some  of  these  are  kaolinite  (sp.  gr.  2.6-2.63), 
quartz  (sp.  gr.  2.65),  calcite  (sp.  gr.  2.72),  chlorite  (sp.  gr.  2.60-2.96), 
serpentine  (sp.  gr.  2.5-2.65),  talc  (sp.  gr.  2.7-2.8),  zeolite  (sp.  gr.  2-2.4), 
limonite  (sp.  gr.  3.5-3.96),  etc.  All  of  these  minerals  and  most  of  the  other 
abundant  undecomposed  minerals,  such  as  feldspar  (sp.  gr.  2.55-2.75), 
which  make  up  great  masses  of  sedimentary  rocks,  have  comparatively 
low  specific  gravities. 

The  second  rule  is  illustrated  by  the  change  from  low  to  high  specific 
gravity  of  the  minerals  where  the  sedimentary  rocks  are  metamorphosed. 
As  just  seen,  the  minerals  which  compose  the  unaltered  sedimentary  rocks 
are  originally  those  of  low  specific  gravity.  Some  of  the  abundant  result- 
ant minerals  in  the  equivalent  metamorphosed  rocks  have  considerably 
higher  specific  gravities,  as,  for  instance,  muscovite  (sp.  gr.  2.76-3),  biotite 
(sp.  gr.  2.7-3.1),  pyroxene  (sp.  gr.  3.2-3.6),  and  amphibole  (sp.  gr.  2.9-3.4), 
and  the  still  heavier  minerals,  garnet  (sp.  gr.  3.15-4.3),  staurolite  (sp.  gr. 
3.65-3.75),  chloritoid  (sp.  gr.  3.52-3.57),  hematite  (sp.  gr.  4.9-5.3),  and 
magnetite  (sp.  gr.  5.168-5.180).  Less  common  heavy  minerals  are  andalusite 
(sp.  gr.  3.16-3.2),  fibrolite  (sp.  gr.  3.23-3.24),  and  chondrodite  (sp.  gr.  3.118- 
3.24).  With  the  above  are  the  lighter  minerals,  quartz  and  feldspar;  but 
even  these  are  quite  as  heavy  as  the  average  of  the  original  minerals. 

It  is  noticeable  in  the  altered  rocks  that  in  proportion  as  deep-seated 
metamorphism  is  advanced  the  heavier  of  the  above  minerals  appear.  In 
the  early  stages  of  the  metamorphism  of  shales,  mica  develops  plentifully, 
and  the  rocks  become  slates.  Where  the  metamorphism  is  more  intense 
the  heavier  minerals,  garnet  and  staurolite,  appear,  the  material  of  the  pre- 
viously developed  micas  being  absorbed  at  the  places  occupied  by  the 
garnet  and  staurolite. 

The  garnet-,  staurolite-,  chloritoid-,  andalusite-,  and  tourmaline-  bearing 
schists  and  gneisses  of  the  Penokee  and  Marquette  districts  of  Michigan 
and  Wisconsin  and  the  Black  Hills  of  South  Dakota,  produced  by  the 


184  A  TREATISE  ON  METAMORPHISM. 

alteration  of  clastic  rocks,  are  perfect  illustrations  of  the  above  changes." 
In  these  rocks  the  acid  feldspars  (sp.  gr.  2.55-2.67)  have  extensively 
altered  into  quartz  (sp.  gr.  2.65)  and  mica  (sp.  gr.  2.76-3.01),  and  therefore 
have  passed  into  minerals  denser  on  the  average  than  those  from  which 
they  were  derived.  Also  the  heavier  minerals,  garnet,  etc.,  have  developed 
on  an  extensive  scale  in  the  more  metamorphosed  varieties. 

When  all  the  minerals  formed  are  taken  into  account  the  average- 
is  as  given.  But  it  is  not  supposed  that  there  are  not  exceptions  to  each  of 
the  rules  that  in  the  upper  physical-chemical  zone  lighter  minerals  form 
and  in  the  lower  zone  heavier  minerals  develop.  Indeed  exceptions  are 
known  to  both.  An  illustration  of  such  exceptions  in  the  upper  zone  is  the 
case  already  mentioned  (see  pp.  181—182),  the  replacement  of  calcium  by 
magnesium.  A  case  of  the  change  from  higher  to  lower  specific  gravity 
in  the  lower  zone  is  the  alteration  of  pyroxene  into  amphibole.  On  the 
average  the  former  is  slightly  heavier,  and  yet  in  the  lower  zone,  under 
both  mass-static  and  mass-mechanical  conditions,  pyroxene  very  generally 
alters  to  amphibole.  Of  course  in  this  transformation  a  change  simultane- 
ously takes  place  in  the  chemical  composition  (and  this  may  have  an  effect 
upon  the  volume  of  the  minerals);  for,  in  general,  pyroxene  contains  a 
greater  proportion  of  calcium  and  less  proportions  of  magnesium  and  iron 
than  the  amphiboles.  If  all  of  the  compounds  concerned  in  the  change 
were  taken  into  account  this  apparent  exception  to  the  rule  of  the  production 
of  compounds  of  high  specific  gravity  in  the  lower  zone  would  probably 
disappear.  In  some  of  the  deepest-seated  schists,  pyroxene  and  not 
amphibole  has  developed,  and  it  is  suspected  that  sufficiently  deep  this  is 
the  rule.  If  this  be  the  case  the  real  meaning  of  the  change  of  pyroxene 
to  amphibole  is,  in  order  that  pressure  shall  become  the  dominant  factor  for 
each  of  the  minerals  as  well  as  for  the  average,  that  the  pressure  must  be 
very  great. 

But  whatever  exceptions  may  be  discovered  in  the  cases  of  individual 
minerals,  the  rules  that  in  the  upper  physical-chemical  zone  the  alterations, 
on  the  average,  result  in  decrease  of  specific  gravity,  and  that  in  the  lower 

a  Irving,  R.  D.,  and  Van  Hise,  C.  R.,  The  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin: 
Mon.  U.  S.  Geol.  Survey,  vol.  19,  1892,  pp.  302-331;  also  vol.  28,  1895,  pp.  448-450,  452-454,  456-459. 
Van  Hise,  C.  R.,  The  pre-Cainbrian  rocks  of  the  Black  Hills:  Bull.  Geol.  Soc.  America,  vol.  1,  1890, 
pp.  222-229. 


SPECIFIC  GRAVITIES  OF  MINERALS  IN  THE  ZONES.          185 

zone  the  alterations  result  in  increase  in  specific  gravity,  are  believed  to 
hold  and  to  be  of  fundamental  importance  in  the  metamorphism  of  rocks." 
This  principle  of  the  development  of  minerals  of  low  specific  gravity 
near  the  surface  and  of  high  specific  gravity  at  depth  has  a  direct  applica- 
tion to  the  crystallization  of  magmas.  From  a  magma  of  a  given  chemical 
composition  there  can  be  little  doubt  that  the  greater  the  depth,  and  there- 
fore the  greater  the  pressure  at  which  crystallization  occurs,  the  higher  the 
average  specific  gravity  of  the  rocks.  In  this  treatise  no  attempt  will  be 
made  to  work  out  the  applications  of  the  rule  to  individual  minerals  and 
rocks,  but  one  illustration  in  reference  to  minerals  and  one  illustration  in 
reference  to  rocks  may  be  cited.  It  is  well  known  that  in  the  lavas  silica 
frequently  crystallizes  in  the  form  of  tridymite  (sp.  gr.  2.28-2.33),  but  that 
in  the  deep-seated  igneous  rocks  quartz  (sp.  gr.  2.65)  only  is  found.  It  is 
believed  that  the  explanation  of  this  fact  is  that  near  the  surface  other 
factors  than  pressure  control  the  crystallization,  and  therefore  that  the  less 
heavy  form  of  crystallized  silica — tridymite — may  be  produced;  and  that  in 
the  lower  zone  pressure  is  the  determinative  factor  in  the  crystallization, 
and  therefore  that  the  heavy  form  of  crystallized  silica — quartz — invariably 
results.  An  illustration  in  reference  to  rocks  is  the  presence  or  absence  of 
glass.  Glass  has  a  lower  specific  gravity  than  the  equivalent  crystallized 
substance.  It  is  well  known  that  where  magmas  crystallize  near  the  surface 
glass  is  a  frequent  product,  and  that  where  magmas  crystallize  deep  below 
the  surface  glass  is  either  very  subordinate  or  absent  altogether.  While 
other  factors  besides  pressure  enter  into  this  result,  it  is  believed  that  the 
frequent  presence  of  glass  near  the  surface  and  the  presence  of  dense 
crystallized  minerals  in  the  equivalent  deeper-seated  rocks  crystallizing 
from  magmas  is  a  very  striking  illustration. of  the  truth  of  the  principle  of 
the  development  of  minerals  and  rocks  of  low  density  where  the  pressure 
is  small  and  of  great  density  where  the  pressure  is  great. 

<*  The  above  conclusions  as  to  the  condensation  of  material  at  considerable  depths  has  an  important 
bearing  upon  Reade's  theory  of  mountain  making.  (Reade,  T.  Mellard,  The  origin  of  mountain 
making,  London,  1886.)  His  explanation  of  the  rise  of  mountains  is  that  the  volume  of  the  thick 
deposits  of  sediments  increases  as  a  consequence  of  the  rise  of  the  isogeotherms.  I  believe  that  possible 
expansion  due  to  this  cause  is  more  than  compensated  in  the  case  of  the  sediments  by  the  mechanical 
bringing  of  the  particles  closer  together  as  the  result  of  pressure,  in  many  instances  to  the  practical 
disappearance  of  the  interspaces,  and  by  the  condensation  of  the  material  itself  by  the  physical- 
chemical  changes  above  explained.  The  condensation  also  has  a  bearing  upon  estimates  of  crustal 
shortening.  In  so  far  as  condensation  occurs,  shortening  of  the  outer  crust  of  the  earth  may  allow 
accommodation  to  a  nucleus  of  decreasing  size  without  crustal  corrugation. 


186  A  TREATISE  ON  METAMORPHISM. 

la  conclusion  of  this  part  of  the  subject  it  may  be  said  that  in  the 
zone  of  katamorphism  the  alterations  are  mainly  controlled  by  the  chemical 
law  that  reactions  take  place  with  liberation  of  heat,  and  this  ordinarily 
results  in  increase  of  volume,  provided  the  compounds  which  form  remain 
as  solids.  In  the  zone  of  anamorphism  the  reactions  are  mainly  controlled 
by  the  physical  law  that  reactions  take  place  with  decrease  of  volume,  and 
this  commonly  results  in  chemical  reactions  with  absorption  of  heat.  In 
the  upper  zone  chemical  law  is  the  determinative  factor  in  the  reactions; 
in  the  lower,  physical  law.  In  the  upper  zone  the  important  chemical 
reactions  are  those  of  oxidation,  carbonation  (involving  desilication),  and 
hydration;  in  the  lower  zone  the  important  reactions  are  those  of  deoxida- 
tion,  silication  (involving  decarbonation),  and  dehydration.  In  the  upper 
zone  the  minerals  are  few  in  number,  of  low  specific  gravity,  and  probably 
of  simple  molecular  structure;  in  the  lower  zone  the  minerals  are 
numerous,  of  high  specific  gravity,  and  probably  of  complex  molecular 
structure. 

If  one  were  to  select  three  words  which  roughly  represent  the  charac- 
teristics of  the  alterations  in  the  belt  of  weathering,  the  belt  of  cementation, 
and  the  zone  of  anamorphism,  these  three  words  would  be,  respectively, 
destruction,  construction,  and  reconstruction. 

GENERAL  CONSIDERATIONS. 

It  is  now  apparent  that  a  geological  classification  of  metamorphism 
dependent  upon  depth  carries  with  it  profound  chemical  and  physical 
significance.  Not  only  is  the  classification  applicable  to  all  parts  of  the 
earth,  but  the  alterations  of  the  zones  of  katamorphism  and  anamorphism 
are  more  fundamentally  different  than  any  distinction  heretofore  made  with 
reference  to  metamorphism.  Also,  the  alterations  of  the  belt  of  weathering 
and  the  belt  of  cementation  of  the  zone  of  katamorphism,  again  dependent 
upon  depth,  are  very  different  so  far  as  the  geological  facts  are  concerned, 
but  are  closely  allied  from  the  chemical  and  physical  point  of  view,  and 
therefore  belong  together  in  a  single  zone. 

It  is  further  to  be  noted  that  the  classification  is  based  upon  one  idea 
throughout.  There  is  no  overlapping,  although  the  different  belts  and 
zones  are  side  by  side  and  grade  into  one  another. 

After  the  universal  geological  applicability  of  the  classification  proposed 


ZONES  OF  METAMORPHISM,  FRACTURE,  AND  FLOW  AGE.      187 

is  understood  and  its  chemical  and  physical  significance  appreciated,  one 
sees  how  partial,  how  overlapping-,  and  how  variable  are  the  criteria  upon 
which  are  based  such  classifications  as  thermo-metamorphism,  hydro- 
metamorphism,  dynamic  metamorphism,  contact  metamorphism,  regional 
metamorphism,  etc. 

RELATIONS    OF    ZONES    OF    KATAMORPIIISM  AND    ANAMORPHISM    TO 
/ONES   OF   FRACTURE  AND  FLOWAGE. 

In  connection  with  metamorphism  in  the  zones  of  katamorphism  and 
anamorphism  it  may  be  recalled  that  the  outer  part  of  the  surface  of  the 
earth  may  be  divided  into  two  zones  upon  a  different  basis."  It  has  been 
shown  from  the  structural  point  of  view  that  we  may  divide  the  rocks  of 
which  we  have  knowledge  into  an  upper  zone  of  rock  fracture  and  a  lower 
zone  of  rock  flowage.  Where  the  rocks  are  subjected  to  deformation  while 
in  the  upper  zone  they  mainly  undergo  mass  fractures  called  bedding  part- 
ings, faults,  joints,  fissility,  etc.  The  deformation  is  accomplished  .but  to  a 
slight  extent  by  fracturing  of  the  individual  particles  and  by  differential 
movement  between  them.  In  the  zone  of  fracture  openings  other  than 
those  formed  by  deformation  may  exist.  In  the  sedimentary  rocks  are 
openings  between  the  grains.  In  the  lavas  there  may  be  gas  openings. 
So  far  as  the  mass-mechai  deal  forces  are  concerned  openings  of  all  of  these 
different  classes  may  persist  indefinitely.  In  the  lower  zone — that  of  rock 
flowage — if  openings  could  be  supposed  to  be  produced  in  any  way,  the 
pressure  is  so  great  that  the  rock  flows  and  fills  them  nearly  completely, 
except  that  water  solutions  and  gases  are  to  a  small  extent  included  in 
minute  cavities.  Rdck  flowage  will  be  subsequently  shown  to  be  mainly 
accomplished  by  innumerable  fractures  of  the  mineral  particles,  by  the 
recrystallizatioii  of  the  mineral  particles,  or  by  a  combination  of  the  two 
processes  in  any  proportion.  Mass  fractures  play  but  a  subordinate  part. 

UPPER  LIMIT  OF  ZONE  OF  FLOWAGE. 

The  depth  at  which  deformation  by  flowage  occurs  depends  upon  many 
factors,  of  which  the  character  of  the  rocks,  the  temperature,  the  water 
content,  and  the  speed  of  deformation  are  of  consequence.  The  more 

o  Van  Hise,  C.  R.,  Principles  of  North  American  pre-Cambrian  geology;  with  an  appendix  on 
flow  and  fracture  of  rocks  as  related  to  structure,  by  L.  M.  Hoskins:  Sixteenth  Ann.  Kept.  TJ.  S.  Geol. 
Survey,  pt.  1,  1896,  p.  589. 


188  A  TREATISE  ON  METAMORPHISM. 

important  factors  which  enter  into  the  character  of  the  rocks  are  the 
strength  and  the  mineral  composition.  The  stronger  the  rock  the  greater 
is  the  depth  at  which  flowage  begins.  The  rocks  the  materials  of  which 
are  refractory,  as,  for  instance,  those  composed  of  quartz  and  feldspar, 
require  a  greater  depth  in  order  that  deformation  ma}-  take  place  by  flow- 
age  than  those  rocks  the  materials  of  which  are  readily  acted  upon 
chemically,  as,  for  instance,  calcite. 

The  higher  the  temperature  the  less  the  depth  in  order  that  deforma- 
tion may  take  place  by  flowage.  Since  the  temperature  increases  normally 
at  the  rate  of  1°  C.  for  30  meters,  and  since  in  consequence  of  orogenic 
movements  and  igneous  intrusions  the  increase  in  temperature  with  depth 
is  often  much  more  rapid  than  this,  heat  is  a  very  important  factor  in  the 
depth  at  which  rock  flowage  occurs. 

Since  rock  flowage  may  be  in  large  measure  by  recrystallization,  and 
recrystallization  is  dependent  to  a  large  extent  upon  the  amount  of  water 
present,  the  greater  the  amount  of  water  the  more  readily  does  deformation 
take  place  by  flowage  and  therefore  the  less  is  the  depth  at  which  flowage 
begins. 

The  speed  of  deformation  is  also  of  very  great  consequence  in  limiting 
the  upper  part  of  the  zone  of  flowage.  The  more  rapid  the  deformation  the 
greater  the  depth  of  the  zone  of  flowage;  the  slower  the  deformation  the 
more  moderate  its  depth.  Speed  of  deformation,  and  therefore  the  time 
consumed  in  a  given  deformation,  is  of  very  great  importance.  It  is  well 
known  that  a  stress  not  sufficient  to  rupture  a  material  or  to  appreciably 
deform  it  within  a  short  time,  if  applied  for  a  long  time  may  produce 
important  flowage  deformation.  The  geologist  must  give  this  factor  of 
time  greater  weight  than  scientists  in  any  other  subject.  How  important 
it  is  may  be  illustrated  by  the  deformation  of  rock"  as  a  result  of  placing  it 
in  an  unusual  position.  In  cemeteries  marble  slabs  have  been  placed 
horizontally  and  supported  at  the  ends;  in  the  course  of  a  score  or  more  of 
years  such  slabs  are  found  to  have  sagged  in  the  middle  a  very  considerable 
amount.  This  is  illustrated  in  the  cemetery  of  Jefferson  City,  Mo.,  where 
a  slab  about  1.8  meters  long,  .9  meter  wide,  and  5.08  centimeters  thick, 

«  Van  Hise,  C.  R.,  Principles  of  North  American   pre-Cambrian  geology:  Sixteenth  Ann.  Rept. 
U.  8.  Geol.  Survey,  pt.  1,  1896,  p.  594. 


DEPTH  OF  ZONE  OF  FLO  WAGE.  189 

suspended  at  the  ends,  has  sagged  3.8  centimeters  in  the  middle.0  If  it  had 
been  attempted  to  bend  the  slab  at  the  outset  to  this  extent,  undoubtedly 
it  would  have  been  raptured.  The  change  in  form  without  rupture  is  pos- 
sible only  by  rock  flowage,  through  a  rupturing  and  differential  movement 
of  the  solid  particles  with  reference  to  one  another,  or  by  solution  and 
redeposition — i.  e.,  by  granulation  or  recry stall ization,  or  by  the  two 
combined. 

On  the  assumptions  (a)  that  the  strength  of  the  rocks  is  the  same  as  at  the 
surface,  (b)  that  the  rocks  are  all  of  the  same  kind,  (c)  that  the  temperature 
is  the  same  as  at  the  surface,  (d)  that  the  water  present  does  not  make  any 
difference  in  the  character  of  deformation,  (e)  that  the  rocks  yield  as  readily 
by  fracture  as  by  flowage,  (f)  that  the  rocks  break  as  readily  by  fracture 
when  the  deformation  is  slow  as  when  it  is  rapid,  and  (g)  that  the  rocks  are 
among  the  strongest,  I  have  calculated  that  the  maximum  depth  of  the 
upper  part  of  the  zone  of  flowage  under  mass-static  conditions  can  not  be 
greater  than  12,000  meters.  All  of  these  assumptions,  except  the  first,  are 
in  favor  of  great  depth  for  the  zone  of  flowage.  It  is  explained  (Chapter 
VIII,  p.  672)  that  where  rocks  are  under  pressure  in  all  directions  the 
rigidity  is  probably  greater  than  at  the  surface.  Therefore  the  assumption 
that  the  rocks  are  no  stronger  below  than  at  the  surface  might  lead  to  too 
small  a  depth.  However,  the  other  assumptions  would  give  too  great  a 
depth,  because  the  great  majority  of  rocks  are  not  nearly  so  strong  as  the 
strongest,  and  many  of  them  have  only  a  small  fraction  of  this  strength; 
because  the  temperature  increases  with  increase  of  depth,  with  orogenic 
movements,  and  with  intrusives;  because  water  is  present  in  considerable 
quantity,  and  where  this  agent  is  available  with  higher  temperatures  the 
rocks  are  deformed  by  flowage  rather  than  by  fracture  (see  p.  188);  and, 
finally,  because  the  rocks  ai-e  ordinarily  deformed  so  very  slowly  that  with 
a  rather  moderate  pressure  the  deformation  takes  place  by  flowage  rather 
than  by  fracture.  I  can  see  no  way  to  determine  to  what  extent  these 
factors  render  the  .maximum  depth  calculated  too  great;  nor  can  any 
estimate  be  made  as  to  how  far  the  factor  (a)  renders  the  maximum  depth 
calculated  too  small ;  but  I  suspect  that  the  various  factors  giving  too  great 
a  depth  are  of  far  greater  consequence  than  the  one  factor  giving  too  small 

a  Winslow,  A.,  An  illustration  of  the  flexibility  of  limestone:  Am.  Jour.  Sci.,  3d  ser.,  vol.  43, 
1892,  pp.  133-134. 


190  A  TKEATISE  ON  METAMORPAISM. 

a  depth.  Since  there  is  no  theoretical  way  accurately  to  evaluate  these 
factors  and  thus  to  calculate  the  maximum  depth  of  the  upper  part  of  the 
zone  of  flowage,  one  can  judge  of  its  real  depth  only  by  observation  in 
mountain  areas  where  deep-seated  rocks  have  been  deformed  when  buried 
under  an  approximately  determinable  thickness  of  rocks  and  afterwards 
have  been  brought  to  the  surface  by  denudation.  From  observation  data 
I  suspect  the  maximum  depth  calculated  is  much  too  great,  perhaps  twice 
too  great  even  for  the  strongest  rocks;  and  for  the  weaker  rocks  it  is  certain 
that  the  alterations  characteristic  of  the  zone  of  flowage  occur  at  depths  but 
a  fraction  of  10,000  or  12,000  meters. 

The  boundary  between  the  zone  of  fracture  and  the  zone  of  flowage  is 
approximately  the  same  as  the  boundary  between  the  upper  and  lower 
physical-chemical  zones  if  indeed  it  is  not  identical  with  it.  We  may 
therefore  say  that  the  upper  physical-chemical  zone,  the  zone  of  katamor- 
phism,  and  the  zone  of  fracture  are  synonymous  terms,  as  are  also  the 
lower  physical-chemical  zone,  the  zone  of  anamorphism,  and  the  zone  of 
flowage.  The  reasons  for  the  correspondence  of  the  zone  of  fracture  with 
the  zone  of  katamorphism,  and  of  the  zone  of  flowage  with  the  zone  of 
anamorphism,  are  clear. 

To  the  bottom  of  the  zone  of  fracture  the  rocks  are  strong  enough  to 
support  themselves,  hence  there  is  not  pressure  in  all  directions  greater  than 
the  strength  of  the  rocks,  and  openings  may  exist.  The  reactions  may 
therefore  take  advantage  of  these  spaces  and  nil  them,  thus  expanding  the 
volume  of  the  rocks  without  lifting  them  or  doing  the  mechanical  work  of 
rupturing  them.  The  openings  which  may  thus  be  utilized  vary  from  those 
of  supercapillary  size,  such  as  bedding  partings,  fault  and  joint  openings,  to 
subcapillary  openings  between  the  individual  grains.  In  order  to  thus  fill 
these  spaces,  no  large  amount  of  work  must  be  done  against  pressure  by  the 
chemical  agents,  but  in  proportion  as  the  spaces  are  filled  it  is  more  and 
more  difficult  for  the  reactions  to  occur  requiring  expansion  of  volume,  as 
an  increased  amount  of  work  must  be  done  against  pressure. 

However,  below  the  bottom  of  the  zone  of  fracture,  in  the  zone  of 
flowage,  the  pressure  in  all  directions  is  greater  than  the  strength  of  the 
rocks.  If  supercapillary  spaces  be  supposed  to  be  present  they  would  be 
closed  by  flow,  unless  this  were  prevented  by  occluded  water  or  some  other 
liquid  or  a  gas  which  could  not  escape.  If  a  reaction  here  occurs  which 


TRANSITIONS  BETWEEN  ZONKS.  191 

demands  expansion  of  volume,  it  would  be  necessary  to  lift  the  entire 
superincumbent  mass  of  rock,  and  this  would  require  a  vast  amount  of 
work.  This  work  chemical  affinity  is  usually  not  sufficiently  strong  to 
accomplish,  therefore  reactions  do  not  take  place  which  give  increased 
volume;  but  on  the  contrary,  the  pressure  forces  reactions  in  the  opposite 
sense  from  those  in  the  upper  zone,  as  a  result  of  which  the  volume  of  a 
material  is  diminished.  If  the  reactions  diminishing  volume  can  be  of  such 
a  character  as  to  liberate  heat,  this  will  occur;  but  frequently,  in  order  to 
produce  a  decreased  volume,  chemical  reactions  must  take  place  which 
absorb  heat.  In  this  paper  on  metamorphism  the  terms  zone  of  katamor- 
phism  and  zone  of  anamorphism  are  used,  as  being  most  serviceable.  In 
structural  work,  however,  the  equivalent  terms  zone  of  fracturing  and  zone 
of  flowage  are  morfe  serviceable  and  therefore  will  there  hold  their  place. 
It  may  be  seen  on  page  167,  also  on  pages  766-768,  that  the  passage 
from  the  zone  of  katamorphism  to  the  zone  of  anamorphism  is  a  gradation 
and  not  an  abrupt  change.  The  same  is  true  of  the  change  from  the 
zone  of  fracture  to  the  zone  of  flowage.  (See  pp.  187-189.)  Therefore, 
whether  the  division  of  the  outer  crust  of  the  earth  into  two  zones  be 
considered  from  the  metamorphic  point  of  view  or  from  the  structural 
point  of  view,  there  is  a  transition  between  the  two. 


CHAPTER  V. 

MINERALS. 

In  the  previous  chapters  I  have  discussed  the  forces  and  agents  of 
alteration,  and  the  general  nature  of  the  alterations  in  the  zones  of 
anamorphism  and  katamorphism,  including  the  two  belts  of  the  latter  zone. 
We  are  now  prepared  to  consider  the  particular  alterations  which  affect  the 
individual  minerals  in  reference  to  these  forces,  agents,  zones,  and  belts. 

SECTION  1.— CHEMICAL  AND  MINERAL  COMPOSITION  OF  THE  KNOWN 

CRUST  OF  THE  EARTH. 

For  convenience  the  outer  part  of  the  crust  of  the  earth  of  which  we 
have  positive  knowledge  will  be  called  the  crust.  Clarke,"  for  the  purpose  of 
considering  the  chemical  composition  of  the  outer  part  of  the  earth,  confines 
this  term  "crust"  to  the  part  of  the  earth  which  extends  from  the  tops 
of  the  mountains  to  10  miles  below  sea  level.  He  thinks  it  fair  to  assume 
that  we  may  infer  the  approximate  composition  of  this  small  part  of  the 
earth  by  the  parts  of  it  which  may  be  observed  at  or  near  the  surface.6  The 
term  "crust"  in  this  treatise  will  be  used  in  the  restricted  sense  of  Clarke- 
But  in  so  using  the  term  there  is  no  intention  to  imply  that  there  is  any 
sharp  division  between  the  crust  and  the  deeper  part  of  the  earth,  to  which 
the  term  "centrosphere"  is  applied. 

Below  is  a  table  which  gives  the  relative  proportions  of  the  twenty-one 
elements  composing  as  much  as  0.01  per  cent  of  the  crust  of  the  earth  as 
above  denned,  including  the  lithosphere,  hydrosphere,  and  atmosphere,  and 
also  their  atomic  weights." 

"Clarke,  F.  W.,  Kelative  abundance  of  the  chemical  elements:  Bull.  U.  S.  Geol.  Survey  No.  78, 
1891,  p.  34. 

» Clarke,  cit.,  pp.  34-37. 

"Clarke,  F.  W.,  Analyses  of  rocks,  laboratory  of  the  United  States  Geological  Survey,  1880-1899: 
Bull.  U.  S.  Geol.  Survey  No.  168,  1900,  p.  15. 
192 


CLASSIFICATION  OF  MINERALS. 

Elements  of  the  eartKs  crust. 


193 


Element. 

Proportion. 

Atomic 
weight. 

Element. 

Proportion. 

Atomic 
weight. 

Oxveen 

Per  cent. 
47.02 

15.  88 

Manganese 

Per  cent. 
.07 

54.57 

Silicon 

28.06 

28.18 

Sulphur         .... 

.07 

31.83 

•Yluminum 

8.  16 

26.91 

Barium 

05 

136  39 

Iron 

4.64 

55.60 

Strontium 

02 

86.95 

Calcium  .........  

3.50 

39.76 

Chromium  .  ...... 

.01 

51.74 

Magnesium 

2.62 

24.10 

Nickel 

01 

58  24 

Sodium 

2.63 

22.88 

Tjithium 

01 

6.97 

Potassium 

2.32 

38.82 

Chlorine 

01 

35  18 

Titanium 

.41 

47.79 

Fluorine 

01 

18  91 

Hydrogen  

.  17 

1.00 

Carbon 

.12 

11.91 

100.00 

Phosphorus  

.09 

•    30.  79 

From  this  table  it  is  seen  that  of  the  metallic  elements  aluminum,  iron, 
magnesium,  calcium,  sodium,  and  potassium  are  the  only  ones  which  may 
be  called  abundant,  and  that  of  the  nonmetallic  elements  oxygen  and 
silicon  are  the  only  two  which  are  abundant,  although  carbon,  sulphur,  and 
chlorine  are  very  important,  and  still  others  of  the  nonmetallic  elements, 
such  as  fluorine  and  phosphorus,  are  of  considerable  consequence.  These 
elements  combined,  or  rarely  alone,  as  they  occur  in  the  natural  state,  are 
called  minerals. 

The  more  important  minerals  are  classified  into  (1)  elements,  (2)  oxides, 
(3)  salts  of  the  binary  acids,  and  (4)  salts  of  the  ternary  acids.  Of  the 
twenty-one  elements  above  given,  only  oxygen,  iron,  nickel,  sulphur,  and 
carbon  occur  in  the  elemental  form,  and  with  the  exception  of  oxygen  the 
amounts  thus  occurring  are  insignificant.  The  free  oxygen  is  mainly  con- 
tained in  the  atmosphere,  but  large  quantities  are  also  included  in  the 
hydrosphere  and  lithosphere.  The  oxides  comprise  both  hydrous  and 
anhydrous  minerals.  Of  the  salts  of  the  binary  acids,  the  sulphides  are 
of  predominant  importance,  but  the  chlorides  and  fluorides  are  of  some 
consequence.  The  salts  of  the  ternary  acids  are  the  predominant  minerals 
of  the  earth's  crust.  They  include  silicates,  carbonates,  titauates,  phos- 
phates, and  sulphates.  These  compounds  are  mentioned  in  the  order  of 
their  importance;  indeed,  the  silicates  are  of  dominating  importance,  but 
MON  XLVII — 04 13 


194  A  TREATISE  ON  METAMORPHISM. 

next  to  them  stand  the  carbonates,  and  the  titanates  and  phosphates  are 
subordinate.  Therefore  the  acids  of  the  silicates  and  carbonates — i.  e., 
silicic  and  carbonic  acids — are  the  great  rock-forming  acids. 

The  natural  combinations  of  the  elements,  so  far  as  they  occur  as 
important  rock-making  constituents,  their  systems  of  crystallization,  chem- 
ical formulae,  molecular  weights,  logarithms  of  molecular  weights,  specific 
gravities,  logarithms  of  specific  gravities,  molecular  volumes  (i.  e.,  molecular 
weights  divided  by  the  specific  gravities),  and  logarithms  of  molecular 
volumes,  are  given  in  tables  below  alphabetically  arranged. 

In  the  tables  the  chemical  formulae  are  usually  those  of  the  smallest 
possible  molecules.  There  is  no  attempt  to  make  the  formulae  correspond 
with  the  real  molecular  structure  of  the  minerals,  since  in  the  present  state 
of  knowledge  it  is.  quite  impossible  to  do  this.  It  therefore  follows  that 
the  molecular  weights,  molecular  volumes,  and  logarithms  of  the  same  in  a 
given  case  are  relative.  The  table  should  not  be  used  to  compare  the 
absolute  molecular  weights  and  molecular  volumes  of  the  different  minerals, 
as  this  would  give  wholly  misleading  results.  For  instance,  actinolite,  the 
second  mineral  in  the  table,  varies  in  the  amount  of  magnesium  and  iron 
so  that  it  may  have  three  special  formula}.  These  formulae  as  written 
make  the  molecular  weight  of  the  mean  molecule  twice  that  of  the 
smallest  and  that  of  the  largest  molecule  four  times  that  of  the  smallest. 
This  results  in  similar  variations  in  the  molecular  volumes  and  also  consid- 
erable variations  in  the  logarithms  Manifestly  there  are  no  such  differ- 
ences as  these.  Probably  the  true  molecular  weights  and  molecular 
volumes,  and  consequently  the  logarithms,  are  very  close  to  one  another. 
However,  the  numbers  given  in  the  table  serve  the  purpose,  as  explained 
011  pages  208-210,  of  calculating  the  changes  of  volumes  when  the  particulai 
varieties  of  the  mineral  actinolite  represented  by  the  formula?  are  trans- 
formed to  other  minerals.  These  remarks  as  between  the  different  varieties 
of  actinolite  apply  equally  well  as  between  other  minerals  and  their  different 
varieties. 

The  letters  D,  Gr,  H,  and  C,  following  formulae  or  specific  gravities, 
signify  that  the  authorities  from  which  the  same  are  taken  are,  Dana, 
Groth,  Hintze,  and  Clarke,  respectively. 


ROCK-MAKING  MINERALS. 


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202  A  TREATISE  ON  METAMORPHISM. 

SECTION  2.— GENERAL  NATURE  OF  ALTERATIONS. 

Minerals  may  be  altered  (1)  without  chemical  change  and  (2)  with 
change  of  chemical  composition. 

ALTERATION  WITHOUT  CHANGE  IN  CHEMICAL  COMPOSITION. 

The  alterations  which  occur  without  changes  in  chemical  composition 
are  (a)  molecular  rearrangement  and  (b)  simple  recrystallization. 

MOLECULAK  REARRANGEMENT. 

Molecular  rearrangement  alone  means  passage  from  one  crystalline 
form  to  another  crystalline  form.  Such  change  of  form  may  result  from 
changed  physical  conditions,  as,  for  instance,  change  in  temperature  or 
pressure  or  movement.  As  an  example  of  molecular  rearrangement  due 
to  change  of  temperature  may  be  mentioned  leucite,  which  crystallizes 
from  a  hot  magma  in  the  regular  system,  but  which  changes  upon  cooling 
to  ordinary  temperatures  to  a  complex  twinned  anisometric  form.  An 
example  of  a  change  due  to  pressure  is  furnished  by  orthoclase,  which  is 
said  for  this  reason  to  alter  to  microcline.0  Molecular  readjustments  such 
as  above  are  simply  changes  of  form,  and  are  therefore  called  paramorphism. 

SIMPLE    RECRYSTALLIZATION. 

Simple  recrystallization  usually  but  probably  not  always  occurs  through 
the  medium  of  a  certain  amount  of  water,  which  is  able  to  take  material  into 
solution  and  deposit  it  from  solution.  Changing  pressure  and  comparatively 
high  temperatures  are  favorable  conditions  for  such  recrystallization.  Per- 
haps the  most  common  example  of  recrystallization  without  chemical 
change  is  that  of  the  transformation  of  amorphous  or  finely  crystalline 
calcium  carbonate  to  crystalline  or  more  coarsely  crystalline  calcium 
carbonate,  such  as  occurs  in  limestones  and  marble.  This  process  has 
been  called  marmorosis.  Another  instance  of  recrystallization  without 
change  in  chemical  composition  which  takes  place,  on  an  extensive  scale,  is 
alteration  of  flinty  or  finely  crystalline  quartz  to  coarsely  crystalline 
quartz. 

ALTERATION  WITH  CHANGE  IN  CHEMICAL  COMPOSITION. 

Alterations  with  chemical  change  may  take  place  (1)  witi.out  the 
addition  or  subtraction  of  material  or  (2)  with  the  addition  or  subtraction 

«  Dana,  J.  D.,  A  system  of  mineralogy,  Descriptive  mineralogy  by  E.  S.  Dana,  Wiley  &  Sons,  New- 
Tone,  6th  ed.,  1892,  p.  318. 


ALTERATIONS  OF  MINERALS.  203 

of  material.  For  either  of  these  changes  the  presence  of  water  is  required 
in  most  instances,  the  alterations  taking  place  through  solution  and  redepo- 
sition,  although  it  is  not  impossible  that  solids  may  act  upon  one  another 
to  an  important  extent  without  the  help  of  water. 

ALTERATION    WITHOUT   ADDITION    OR   SUBTRACTION   OF   MATERIAL,. 

Iii  the  changes  which  occur  under  this  case  the  material  moves  only 
short  distances.  Such  changes  may  be  (a)  a  crystallization  of  an  amor- 
phous substance  or  (b)  interior  alteration  of  mineral  particles. 

An  instance  of  the  crystallization  of  an  amorphous  substance  is 
furnished  by  the  devitrification  of  glass.  In  this  alteration  the  uniform 
homogeneous  solid  glass  changes  into  a  heterogeneous  crystalline  solid,  the 
different  mineral  particles  of  which  have  differing  compositions.  This 
involves  segregation  of  the  different  elements  in  various  proportions  into 
the  different  minerals.  It  is  therefore  clear  that  the  materials  have  moved 
very  short  distances. 

Interior  alteration  of  mineral  particles  is  effected  by  the  change  of 
one  mineral  into  two  or  more  minerals.  This  is  illustrated  by  the  change 
of  pyrope  into  enstatite,  spinel,  and  quartz;  the  change  of  pyrope  into 
hypersthene,  spinel,  and  quartz;  the  change  of  spodumene  into  eucryptite 
and  albite;  the  change  of  almandite  into  hypersthene,  spinel,  and  quartz; 
and  the  change  of  titanite  into  perovskite  and  quartz. 

ALTERATION   WITH    ADDITION    OR   SUBTRACTION    OF   MATERIAL. 

The  changes  which  take  place  with  the  addition  or  subtraction  of 
material  may  vary  from  those  which  involve  the  slightest  addition  or 
subtraction  to  complete  substitution.  The  added  material  may  come  from 
afar  or  from  the  adjacent  mineral  particles.  The  subtracted  material  may 
enter  into  an  adjacent  mineral  particle  or  may  be  transported  great 
distances  before  entering  into  a  new  mineral.  Reactions  between  adjacent 
minerals  may  produce  new  minerals.  Two  or  more  minerals  may  unite  to 
produce  a  single  mineral.  For  example,  olivine  and  quartz  may  pass  into 
anthophyllite;  nephelite  and  halite  into  sodalite;  albite  and  halite  into 
rnarialite.  Or  two  or  more  minerals  may  unite  to  produce  two  or  more 
new  minerals.  For  example,  rutile  and  magnetite  may  pass  into  ilmenite 
and  hematite;  diopside  and  magnetite  into  tremolite  and  calcite;  sahlite, 
siderite,  and  magnesite  into  actinolite  and  calcite;  augite,  siderite,  and 


204  A  TREATISE  ON  METAMORPHISM. 

magnesite  into  hornblende  and  calcite;  or  the  reverse  of  this,  hornblende 
and  calcite  into  augite,  siderite,  and  magnesite. 

The  changes  here  belonging  are  by  far  the  most  numerous  and  impor- 
tant of  the  various  classes;  indeed,  are  vastly  more  important  than  all  of 
the  other  classes  together.  By  far  the  greater  number  of  reactions  written 
out  on  the  succeeding  pages  for  the  alterations  of  the  various  minerals  fall 
under  this  heading. 

The  more  important  of  these  alterations,  considered  from  the  point  of 
view  of  the  nonmetallic  elements,  may  be  classified  into: 

(I)  Oxidation.  (2)  Deoxidation. 
(3)  Hydration.  (4)  Dehydration. 
(5)  Carbonation.  (6)  Decarbonation. 
(7)  Silication.  (8)  Desilication. 
(9)  Silicification.  (10)  Desilicification. 

Less  important  reactions  are: 

(II)  Sulphidation.  (12)  Desulphidation. 
(13)  Sulphation.  (14)  Desulphation. 
(15)  Titanation.  (16)  Detitanation. 
(17)  Phosphation.  (18)  Dephosphation. 
(19)  Chloridation.  (20)  Dechloridation. 
(21)  Fluoridation.  (22)  Deflnoridation. 
(23)  Boration.  (24)  Debor?tion. 

A  number  of  these  reactions  are  of  small  consequence  so  far  as  the 
alterations  of  rocks  are  concerned;  but  all  are  important  with  reference  to 
the  development  of  minerals,  and  especially  in  reference  to  economic 
products.  This  phase  of  the  subject  in  reference  to  the  metallic  products 
is  treated  in  Chapter  XII. 

(1)  Oxidation  is  the  addition  of  oxygen.     Frequently  the  added  oxygen 
is  substituted  for  another  element,  often  sulphur. 

(2)  Deoxidation  is  the  subtraction  of  oxygen.     Often  the  subtracted 
oxygen  is  replaced  by  another  element — for  instance,  sulphur. 

(3)  Hydration  is  the  addition  of  water,  producing  hydroxides. 

(4)  Dehydration  is  the  subtraction  of  water  from  hydroxides.     When 
carried  to  completion,  anhydrous  compounds  are  formed. 

(5)  Carbonation  is  the  union  of  carbonic  acid  and  base,  or  the  substi- 
tution of  carbonic  acid  for  another  combined  acid,  in  either  case  producing 
carbonates.     The  oxide  with  which  carbonic  acid  most  frequently  unites  is 


CLASSIFICATION  OF  ALTERATIONS.  205 

iron  oxide.  Carbonic  acid  may  replace  any  of  the  other  ternary  rock- 
forming  acids,  including  silicic,  titanic,  and  phosphoric,  and  thus  become 
•united  with  any  of  the  important  bases.  The  carbonation  of  the  silicates 
is  of  fundamental  importance.  The  carbonation  of  the  titanates  and  phos- 
phates is  unimportant. 

(6)  Decarbonation  is  the  separation  of  carbonic  acid  from  a  base  with- 
out the  addition  of  other  compounds,  or  with  the  substitution  of  another 
acid  for  the  carbonic.     The  most  frequent  substituted  acid  is  silicic. 

(7)  Silication  is  the  union  of  silicic  acid  and  base,  or  the  substitution 
of  silicic  acid  for  a  combined  acid,  in  either  case  producing  silicates.     The 
only  important  oxide  with  which  silicic  acid  unites  as  a  rock-forming  con- 
stituent is  iron  oxide      Silicic  acid  may  replace  carbonic,  titanic,  or  phos- 
phoric acid,  thus  becoming  united  with  any  of  the  bases  with  which  it  can 
combine.     The  silication  of  the  carbonates  is  of  fundamental  importance. 
The  silication  of  the  titanates  and  phosphates  is  unimportant. 

(8)  Desilication  is  the  separation  of  silicic  acid  and  bases  without  the 
addition  of  other  compounds,  or  with  the  substitution  of  another  acid  for 
the  silicic  acid.     The  most  frequent  acid  substituted  is  carbonic. 

(9)  Silicification  involves    the  addition  of  silica  without  union  with 
bases.     The  added  silica  may  or  may  not  replace  other  compounds. 

(10)  Desilicification  involves  the  subtraction  of  free  silica.     The  sub- 
tracted silica  may  or  may  not  be  replaced  by  other  compounds. 

(11)  Sulphidation    is  the    union   of    sulphur    with  a   metal   forming 
sulphides.     Added  sulphur  may  be  substituted  for  another  element,  usually 
oxygen. 

(12)  Desulphidation  involves  the  subtraction  of  sulphur.     Generally 
the  subtracted  sulphur  is  replaced  by  another  element,  usually  oxygen. 

(13)  Sulphation   is    the  union    of  sulphuric   acid    with   base  or  the 
substitution  of  sulphuric  acid  for  another  combined  acid,  in  either  case 
producing  sulphates. 

(14)  Desulphation  is  the  separation  of  sulphuric  acid  and  base,  or  the 
substitution  of  another  acid  for  the  sulphuric. 

(15)  Titanation  is  the  union  of  titanic  acid  with  base,  or  the  substitution 
of  titanic  acid  for  another  combined  acid,  in  either  case  producing  titauates. 

(16)  Detitanation  is  the  separation  of  titanic  acid   and  base,  or  the 
substitution  of  another  acid  for  the  titanic. 


206  A  TREATISE  ON  METAMORPHISM. 

(17)  Phosphation  is  the  union  of  phosphoric  acid  with  base,  or  the 
substitution  of  phosphoric  acid  for  another  combined  acid,  in  either  case 
producing  phosphates. 

(18)  Dephosphation  is  the  separation  of  phosphoric  acid  and  base,  or 
the  substitution  of  another  acid  for  the  phosphoric  acid. 

(19)  Chloridation  is  the  addition  of  chlorine,  forming  chlorides. 

(20)  Dechloridation  is  the  subtraction  of  chlorine,  destroying  chlorides. 

(21)  Fluoridation  is  the  addition  of  fluorine,  forming  fluorides. 

(22)  Defluoridation  is  the  subtraction  of  fluorine,  destroying  fluorides. 

(23)  Boration  is  the  union  of  boric  acid  with  base,  or  the  substitution 
of  boric  acid  for  another  combined  acid,  in  either  case  producing  borates. 

(24)  Deboration  is   the   separation   of  boric   acid   and   base,  or  the 
substitution  of  another  acid  for  the  boric. 

GENEKAL   STATEMENTS. 

The  foregoing  processes  are  seen  to  be  in  pairs,  in  each  case  one  of  a 
pair  being  the  reverse  of  the  other.  That  is,  deoxidation  is  the  reverse  of 
oxidation,  dehydration  is  the  reverse  of  hydration,  etc.  Moreover,  one  of 
the  processes  of  a  pair  is  in  several  of  the  cases  frequently  the  complement 
of  that  of  another  pair.  To  illustrate,  the  processes  of  the  following  pairs 
are  often  complementary  of  each  other,  viz,  oxidation  and  desulphidation, 
sulphidation  and  deoxidation,  carbonation  and  desilication,  silication  and 
decarbonation.  By  complement  is  meant  that  one  takes  place  simultaneously 
with  the  other,  and  that  the  two  may  really  be  one  chemical  reaction.  In 
such  a  case  the  change  may  be  considered  from  either  of  two  points  of  view. 
To  illustrate,  the  process  of  carbonation  may  be  also  a  process  of  desilication, 
and  the  process  of  silication  may  be  also  a  process  of  decarbonation. 
In  general  the  process  is  named  on  the  basis  of  the  substance  added  rather 
than  that  subtracted,  for  such  substance  is  the  active  agent  which  drives 
off"  the  other  and  takes  it  place.  It  has  been  shown  (pp.  168,  170-181) 
that  for  several  reactions  one  of  a  pair  is  particularly  characteristic  for  one 
of  the  zones  of  metamorphism.  To  illustrate,  oxidation  and  its  complement 
desulphidation,  carbonation  and  its  complement  desilication,  and  hydration 
are  particularly  characteristic  of  the  zone  of  katamorphism ;  sulphida- 
tion and  its  complement  deoxidation,  silication  and  its  complement  decar- 
bonation, and  dehydration  are  particularly  characteristic  of  the  zone  of 
anamorphism. 


MINERALS.  207 

SECTION  3.— ROCK-MAKING  MINERALS. 

MANNER  OF  TREATMENT. 
GENERAL   STATEMENTS. 

In  this  treatise  only  the  principal  rock-forming  minerals  will  be  consid- 
ered. The  point  of  view  is  not  that  of  mineralogy  but  that  of  metamorphism. 
So  far  as  it  seems  advisable  without  too  much  repetition,  I  shall  consider 
each  mineral  in  reference  to  the  following: 

(1)  Its  composition,  crystallization,  specific  gravity,  and  source,  so  far 
as  it  is  a  rock-making  mineral ;  but  its  occurrence  in  veins  will  not  be 
considered. 

(2)  The  minerals  into  which  it  may  pass,  giving  their  crystallizations, 
specific  gravities,  and  compositions. 

(3)  The  chemistry  and  physics  of  the  processes  of  change,  including 
the  volume  relations. 

(4)  The  natural  conditions  under  which  the  changes  occur,  and  the 
causes  of  the  changes. 

With  many  minerals  this  outline  can  be  carried  out  nearly  to  com- 
pletion. With  others  the  present  state  of  knowledge  is  such  that  it  can 
be  only  very  incompletely  done.  Consequently  there  is  great  variation 
in  the  satisfactoriuess  of  the  discussion  of  the  different  minerals.  When 
the  treatment  of  each  of  the  minerals  from  these  various  points  of  view 
can  be  carried  out  we  shall  have  an  interlocking  system  by  which  each 
mineral  is  considered  in  its  most  important  metamorphic  connections. 
To  a  certain  extent  the  plan  involves  repetitions,  but  in  each  case  the 
important  facts  which  concern  an  individual  mineral  are  brought  together. 
The  method  of  treatment  proposed  seems  advisable,  for  many  minerals 
are  both  primary  and  secondary,  and  only  by  considering  each  mineral 
from  both  points  of  view  is  it  possible  to  understand  the  causes  of  the 
changes  as  well  as  the  changes  themselves.  Ordinarily  the  latter  only  are 
considered.  When  one  of  the  sources  of  a  mineral  is  the  alteration  of 
another  the  exact  reactions  concerned  in  the  change  are  not  given  under 
the  former,  but  may  be  found  by  referring  to  the  latter  mineral  which  is 
mentioned  as  its  source.  Ordinarily,  however,  qualitative  statements  are 
made.  To  illustrate,  a  source  of  limonite  is  siderite.  The  reactions  involved 
in  this  change  are  to  be  found  under  siderite,  not  under  limonite;  but  under 


208  A  TREATISE  ON  METAMOKPHISM. 

the  latter  mineral  the  statement  is  made  that  the  change  generally  involves 
liberation  of  heat  and  decrease  of  volume.  But  when  a  mineral  is  derived 
by  precipitation  from  a  solution,  or  results  by  the  combination  of  several 
minerals,  it  is  necessary  to  consider  the  chemistry  and  physics  of  the  change 
in  connection  with  the  sources,  for  otherwise  this  important  part  of  the 
history  of  metamorphism  of  minerals  would  be  omitted. 

In  discussing  the  sources  of  a  mineral  when  it  is  derived  from  other 
minerals  the  natural  conditions  of  the  alterations  are  not  given,  but  may  be 
found  by  referring  to  the  minerals  from  which  the  one  under  discussion 
is  derived.  But  where  a  mineral  is  derived  by  the  interaction  or  union  of 
several  other  minerals  the  natural  conditions  are  discussed  under  the  source 
of  the  mineral,  for  otherwise  this  part  of  the  subject  would  be  omitted. 

As  this  treatise  was  originally  planned  it  was  designed  to  include  the 
heat  and  volume  changes  with  the  chemical  reactions.  But  with  the 
present  state  of  knowledge  of  the  heat  relations  in  chemical  transformations 
the  first  has  been  found  impracticable.  While  very  few  quantitative 
results  can  be  given,  in  many  cases  it  is  possible  to  make  a  qualitative 
expression  of  the  heat  reaction.  To  illustrate,  the  heat  of  combination  of 
calcium  is  far  greater  than  that  of  iron  in  all  analogous  compounds  in 
which  determinations  have  been  made;  but  such  determinations  have  not 
been  made  with  reference  to  the  silicates.  Where  calcium  is  replaced  by 
iron  in  the  alteration  of  the  silicates  it  is  inferred  that  a  considerable  amount 
of  heat  is  absorbed,  though  the  exact  amount  can  not  be  specified.  Vice 
versa,  where  iron  is  replaced  by  calcium,  a  considerable  amount  of  heat  is 
liberated.  Of  course,  in  each  reaction  the  other  chemical  combinations 
which  occur  simultaneously  should  be  considered,  for  they  constitute  a  part 
of  the  chain,  and  in  obtaining  a  correct  end  result  their  effects  are  vital.  If, 
for  instance,  a  salt  of  iron  and  a  salt  of  calcium  interchange  acids,  no  general 
statement  can  be  made  as  to  the  heat  reaction.  Therefore,  if  at  the  same 
time  the  iron  replaces  the  calcium  the  calcium  unites  with  an  acid  which 
was  before  in  combination  with  the  iron,  the  inference  above  given  as  to 
absorption  of  heat  can  not  be  made. 

For  the  calculation  of  the  volume  changes,  the  equations  of  the  chem- 
ical reactions  written  out  by  me  and  the  specific  gravities  of  the  minerals, 
taken  from  the  standard  Mineralogies,  were  turned  over  to  Mr.  A.  T.  Lincoln, 
who  made  the  numerical  computations.  Subsequently  Mr.  R.  M.  Chapman 


CALCULATION  OF  VOLUME  RELATIONS.  209 

repeated  the  work  in  order  to  verify  it.  The  following  well-known  principle 
was  employed: 

The  volume  of  the  original  compound  is  to  the  volume  of  the  compound 
produced  directly  as  their  molecular  iveights  and  indirectly  as  their  specific 
gravities. 

Under  this  general  principle  are  two  cases : 

Case  1.  Where  one  solid  compound  alters  into  another  solid  compound. 
This  case  is  illustrated  by  the  well-known  changes  of  limestone  to  dolomite. 
In  this  change  we  have  2CaCO3  replaced  by  MgCa(CO3)2.  The  molecular 
weight  of  2CaCO3  is  198.62.  The  molecular  weight  of  MgCa(CO3)2  is 
182.96.  The  specific  gravity  of  calcite  may  betaken  as  2.7135;  of  dolomite, 
as  2.85.  The  compound  proportion  is  therefore  as  follows: 

v    Vl      198.62:182.96 

2.  85  :     2.  7135 

or  the  volume  of  the  dolomite  is  87.70  per  cent  of  that  of  the  calcite; 
or,  therefore,  there  is  a  decrease  in  volume  of  12.30  per  cent. 

Case  2.  This  has  three  phases:  (a)  where  two  or  more  solid  compounds 
unite  to  produce  a  single  solid  compound ;  (b)  where  a  single  solid 
compound  breaks  up,  producing  two  or  more  compounds,  and  (c) 
1  where  two  or  more  solid  compounds  unite  to  produce  two  or  more  solid 
compounds.  In  this  case  the  method  of  calculation  is  slightly  different 
from  case  1.  The  molecular  weights  of  each  of  the  compounds  represented 
in  the  equations  are  divided  by  the  specific  gravities  of  the  respective 
compounds.  This  gives  their  relative  volumes.  In  phase  (a)  the  volume 
of  the  resultant  single  compound  is  divided  by  the  sum  of  the  volumes  of 
the  producing  compounds,  and  this  gives  the  percentage  of  change.  In 
phase  (b)  the  sum  of  the  volumes  of  the  resultant  compounds  is  divided  by 
the  volume  of  the  original  compound.  In  phase  (c)  the  sum  of  the  volumes 
of  the  resultant  compounds  is  divided  by  the  sum  of  the  volumes  of  the 
original  compounds.  These  different  phases  are  so  similar  in  method  that 
it  is  necessary  only  to  illustrate  one  of  them.  The  first  phase  is  illustrated 
by  the  formation  of  wollastonite  by  the  union  of  calcite  and  quartz,  the 
reactipn  being: 

CaCO3+SiO2=CaSiOs+CO2. 

The  molecular  weights  of  the  three  solid  compounds  are,   respectively, 
MON  XLVII — 04 14 


210  A  TREATISE  ON  METAMORPHISM. 

99.31,  59.94,  and  115.58.     Their  specific  gravities  are  2.7135,  2.6535,  and 
2.85,  respectively.     The  volume  of  the  wollastouite  is,  therefore: 

115.58  .  /99.31      .  59.94     \_ 
^85—  ^  ^7135+  "276535  J~ 

That  is,  the  decrease  in  volume  in  this  case  of  silication  of  calcite  is  31.5 
per  cent. 

In  order  to  expedite  the  laborious  numerical  calculations  of  the  volume 
relations  for  the  very  numerous  alterations,  Mr.  Lincoln  completed  the  table 
on  pp.  195-201  by  adding  the  molecular  weights,  the  logarithms  of  the 
molecular  weights,  the  logarithms  of  the  specific  gravities,  the  molecular 
volumes,  and  the  logarithms  of  the  molecular  volumes  of  each  of  the  min- 
erals. These  determinations  have  been  carefully  verified  by  Mr.  Chapman, 
and  may  be  used  to  check  the  volume  changes  given  in  the  succeeding 
pages,  and  also  to  make  additional  volume  calculations. 

In  calculating  the  volume  relations,  unless  otherwise  specified,  the 
compounds  on  both  sides  of  the  equations  are  regarded  as  solid  except  those 
which  by  themselves  independent  of  the  solvents  are  liquids  or  gases,  such 
as  H2O  and  C02.  All  such  compounds  are  supposed  to  be  added  in  the 
solutions  or  to  be  taken  away  by  the  solutions,  and  therefore  are  not  taken 
into  account  in  the  volume  calculations.  In  general  these  liquid  and 
gaseous  compounds  do  undoubtedly  escape  in  large  measure,  although  in 
some  cases  they  are  confined  as  inclusions  within  the  minerals  formed. 
(See  p.  678.) 

Where  -j-k  is  added  to  the  equation,  this  signifies  that  heat  is  liberated; 
where  — k  appears,  this  means  that  heat  is  absorbed  by  the  reaction. 

No  claim  is  made  that  the  equations  which  ^re  written  in  the  following 
pages  exactly  represent  the  changes  that  take  place  in  the  alterations  of 
the  various  minerals  into  other  minerals  Indeed,  the  probability  is  that 
not  half  exactly  represent  the  facts;  for  the  great  majority  of  the  reactions 
are  more  complicated  than  written,  and  in  many  cases  substances  in  the 
solutions  or  as  solids  not  taken  into  account  are  concerned.  Since  these 
are  the  facts,  the  question  may  be  asked  why  the  equations  are  written. 

The  answer  is,  first,  that  at  some  time  the  attempt  must  be  made  to  givr 
a  first  approximation  to  quantitative  exactness  in  the  alteration  of  minerals. 
The  equations  found  on  the  following  pa^es  represent,  such  an  attempt. 
Before  the  appearance  of  this  treatise  scarcely  more  than  a  score  of  mineral 


CALCULATION  OF  VOLUME  RELATIONS.  211 

alterations  have  been  expressed  by  chemical  equations,  and  in  fewer  still 
have  the  volume  relations  been  calculated.  Second,  the  imperfect 
equations  herein  contained  will  be  sure  to  lead  to  closer  investigations  of 
the  nature  of  the  alterations,  and  to  improved  equations  representing  them. 
Thus  the  progress  of  science  will  be  promoted  by  the  set  of  equations  here 
given,  even  if  the  great  majority  of  them  are  defective.  Third,  it  is 
believed  that  when  a  more  nearly  correct  set  of  equations  is  written  it  will 
be  found  that  the  large  majority  of  the  equations  herein  contained  substan- 
tially represent  the  facts,  and  consequently  that  the  volume  changes  are  in 
most  cases  roughly  approximate.  Many  of  them  may  be  changed  by  a 
few  per  cent  one  way  or  the  other;  but  the  sign  of  few  will  be  changed, 
and  this  is  the  fundamental  point  in  reference  to  the  zones  in  which  the 
alterations  occur. 

The  weakest  point  in  the  accuracy  of  the  volume  reactions  is  not  tound 
in  the  chemical  equations,  but  in  the  inexactness  of  the  specific  gravities  of 
the  minerals  as  given  in  the  text-books.  For  most  minerals  there  is  a  con- 
siderable range  of  specific  gravity  given;  and  with  the  exception  of  one  or 
two  minerals,  such  as  calcite  and  quartz,  it  is  impossible  to  ascertain  the 
exact  specific  gravity  of  the  pure  minerals.  In  the  table  the  mean  between 
the  two  best  determined  extremes  is  given  as  the  best  approximation 
available  of  the  specific  gravities  of  the  pure  minerals.  For  most  minerals 
these  extremes  are  taken  from  Dana's  System  of  Mineralogy. 

The  facts  as  to  the  occurrences  and  alterations  of  the  various  minerals 
given  in  the  following  pages  are  largely  taken  from  the  standard  text- 
books .  of  mineralogy  and  petrology,  and  especially  from  Dana's  great 
System  of  Mineralogy.  The  information  available  is  especially  imperfect 
as  to  the  manner  in  which  the  complex  minerals,-  and  particularly  the 
complex  silicates,  break  up  into  simpler  compounds  in  the  belt  of  weather- 
ing. As  explained  fully  in  the  following  chapter,  this  is  a  general  process. 
For  the  better  known  of  these  changes  equations  are  written,  but  no 
attempt  is  made  to  express  by  equations  the  manner  in  which  many  of  the 
minerals  decompose  and  degenerate,  because  so  little  exact  information  is 
available  upon  which  to  base  such  equations. 

As  already  stated,  only  those  minerals  will  be  considered  which  are 
important  rock-making  constituents.  It  is  impracticable  at  the  present  time 
to  consider  the  physical-chemistry  of  the  rarer  minerals. 


212  A  TREATISE  ON  METAMORPHISM. 

l 

Following  the  ordinary  classification,  the  abundant  rock-making 
constituents  may  be  considered  under  the  headings:  Native  Elements, 
Sulphides,  Fluorides,  Oxides,  Carbonates,  Silicates,  Titanates,  Phosphates, 
and  Sulphates. 

NATIVE  ELEMENTS. 

GRAPHITE. 

Graphite: 

Crystallized  carbon  (C). 

Rhombohedral. 

Sp.  gr.  2.09-2.23;  av.  2.16. 

occurrence — Graphite  occurs  as  a  very  widely  disseminated  constituent 
in  the  extremely  metamorphosed  sedimentary  rocks,  which  in  their  original 
condition  contained  carbonaceous  material.  It  is  especially  prevalent  in 
scales  in  the  marbles,  schists,  and  gneisses.  In  some  instances  the  original 
beds  were  so  heavily  carbonaceous  as  to  give  considerable  layers  a  large 
percentage  of  which  is  graphite.  Such  layers  are  illustrated  by  the 
graphitic  shales  of  Worcester,  Mass."  Graphite  occurs  to  some  extent  with 
the  very  hard  anthracite  coals,  a  part  of  the  carbon  having  passed  over  to 
the  graphitic  condition.  Such  graphitic  coals  occur  in  the  Rhode  Island 
coal  field.6  The  reaction  producing  graphite  as  a  metamorphic  mineral 
requires  great  pressure  and  takes  place  with  decrease  in  volume.  This 
mineral  in  the  sedimentary  rocks  is  therefore  a  product  of  the  zone  of 
anamorphism. 

Graphite  is  said  to  occur  as  an  original  constituent  in  some  basaltic 
rocks.  During  the  alterations  of  carbonaceous  rocks  the  hydrocarbon  com- 
pounds, as  gases,  oils,  and  bitumen,  wander  widely  in  the  solutions.  In 
some  cases  such  compounds  are  deposited  in  the  openings  of  original  rocks 
Later  these  compounds  may  be  altered  to  graphite,  and  yet  the  carbon  not 
be  an  original  constituent  of  the  magma  from  which  the  rocks  crystallized. 

Alterations — Alterations  of  graphite  are  not  recorded,  but  it  is  by  no 
means  certain  that  this  mineral  is  not  very  slowly  oxidized  under  favorable 
conditions  in  the  belt  of  weathering. 

THE    SULPHIDES. 

The  sulphides  which  are  important  as  rock-making  minerals  are 
pyrrhotite,  pyrite,  and  marcasite.  Many  other  sulphides  are  important  in 

a  Perry,  J.  H.,  Note  on  a  fossil  coal  plant  found  at  the  graphite  deposit  in  mica-schists  at  Worcester, 
Mass.:  Am.  Jour.  Sci.,  3d  ser.,  vol.  29,  1885,  pp.  157-158. 

&Shaler,  N.  S.,  Woodworth,  J.  B.,  and  Foerste,  A.  F.,  Geology  of  the  Narragansett  Basin:  Mon. 
U.  8.  Geol.  Survey,  vol.  33,  1899,  p.  82. 


OCCURRENCE  OF  PYRRHOTITE,  PYRITE,  AND  MARCASITE.      213 

the  genesis  of  ore  deposits.     These,  however,  will  be  considered  only  in  the 
chapter  on  that  subject. 

PYRRHOTITE,  PYRITE,  AND   MARCASITE. 

Pyrrhotite: 

Fe5S6  to  Fe15S16;  chiefly  FeuS,,. 

Hexagonal. 

Sp.  gr.  4.58-4.64. 
Pyrite: 

FeS2. 

Isometric. 

Sp.  gr.  4.95-5.10. 
Marcasite: 

Fe82. 

Orthorhombic. 

Sp.  gr.  4.85-4.90. 

occurrence. — Pyirliotite,  pyrite,  and  marcasite  are  very  widespread  acces- 
sory minerals,  occurring  in  rocks  of  all  ages  and  all  kinds.  So  far  as  known, 
these  minerals  arc  not  abundant  original  pyrogenic  constituents,  although 
they  frequently  are  found  along  the  contact  between  intrusive  and  other 
rocks,  occurring  in  both  the  intrusive  and  the  intruded  rocks.  Pyrrhotite 
is  an  original  mineral  in  meteorites.  These  minerals  extensively  form  in 
rocks  in  volcanic  districts  through  the  action  of  solutions  of  hydrogen 
sulphide  and  other  sulphide  solutions  upon  iron  salts.  As  secondary  minerals 
in  the  sedimentary  rocks,  and  to  a  less  extent  in  the  igneous  rocks,  the 
sulphides  are  extensively  formed  through  the  reducing  action  of  organic 
compounds  upon  the  sulphites  and  sulphates,  especially  the  latter,  and  par- 
ticularly iron  sulphate.  Such  reduction  is  characteristic  of  the  belt  of 
cementation  and  the  zone  of  anamorphism ;  but  in  the  latter  zone  pyrrhotite 
or  pyrite,  rather  than  marcasite,  probably  forms. 

The  reducing  agent  of  the  sulphites  and  sulphates  may  be  either  a 
solid  organic  compound  or  one  of  its  gaseous  products  of  decomposition, 
such  as  carbon  monoxide  (CO)  and  carburetted  hydrogen  (CH4).  If  the 
reducing  agent  be  taken  as  CO,  the  reaction  for  pyrite  and  marcasite 

may  be: 

2FeSO4+7CO=FeS2+FeCO5+6CO2+ka 

and  for  pyrrhotite: 

12FeSO4+45CO=Fe11S,2+FeCO.)+45COj-|-k. 

If  the    reducing   agent  were    taken  as    carbon,  similar  results  would   be 
obtained,  except  that  the  amount  of  CO2  would  be  less.     This  action,  while 

"See  page  210. 


214  A  TREATISE  ON  METAMORPH1SM. 

ordinarily  called  a  reduction,  is  reduction  so  far  as  the  iron  sulphate  is 
concerned,  but  is  oxidation  so  far  as  the  carbon  compound  is  concerned, 
and  hence  the  explanation  of  the  liberation  of  heat. 

Pyrite,  raarcasite,  and  pyrrhotite  are  also  doubtless  produced  by  the 
action  of  soluble  sulphides  upon  the  iron  oxides  or  iron  salts.  In  the 
change  from  crystallized  Fe203  (hematite)  to  FeS2  (in  the  form  of  pyrite), 
the  volume  increases  56.14  per  cent. 

Alterations. — The  first  alteration  to  be  considered  is  that  of  marcasite  into 
pyrite.  In  this  alteration  there  is  recrystallization,  an  increase  of  symmetry, 
a  decrease  of  2.98  per  cent  in  volume,  but  no  change  in  chemical  compo- 
sition. The  heat  effect  is  undetermined,  but  probably  heat  is  liberated. 

The  mineral  pyrrhotite  by  recrystallizatiou  passes  into  pyrite.  This 
change  may  occur  in  volcanic  districts  by  the  action  of  hydrogen  sulphide 
upon  the  pyrrhotite,  the  reaction  perhaps  being: 

FeuSl2+10H2S=llFeS2+10H2. 

Ill  this  change  the  volume  is  increased  21.13  per  cent. 

The  minerals  pyrite  and  marcasite  may  by  oxidation  pass  directly 
into  (1)  hydrated  sesquioxide  of  iron,  of  which,  ordinarily,  limonite  (not 
crystallized;  sp.  gr.  3.80)  is  the  most  common  kind;  (2)  magnetite  (isomet- 
ric; sp.  gr.  5.174);  (3)  ferrous  sulphate,  which  may  be  removed  in  solution, 
or  (4)  may  be  decomposed  by  further  oxidation,  either  at  the  place  of 
formation  or  elsewhere,  after  a  longer  or  shorter  time,  into  hydrated  sesqui- 
oxide of  iron,  ordinarily  limonite.  The  reactions  for  marcasite  and  pyrite 
may  be  as  follows,  assuming  in  each  case  that  the  sulphur,  or  a  part  of  it, 
is  also  oxidized: 

(1 )  4FeS2+22O+3H2O=2Fe2O,.3H20+8SO2+k. 

(2)  3FeS,+16O=FesO4+6SO,+k,  or 
3FeS!1+4H2O+4O=Fe,O4+4H.1S+2SO2+k. 

(3)  FeS2+6O=FeSO4+SO2+k,  or 
FeSj+3O+H,O=FeSO4+H2S+k. 

(4)  4FeSO4+2O+7H2O=2Fe,O3.3HsO+4HaSO4+k. 

As  shown  in  Chapter  XI,  on  "Ore  deposits,"  pyrite  and  marcasite  also 
alter  to  hematite  without  oxidation  by  the  reaction  of  an  alkaline  carbonate. 
The  alteration  of  common  pyrrhotite  into  magnetite  aud  limoiiite  may 
be  written  as  follows: 

(5)  3Fe11S1,+1160=llFe,04+36SO.,+k,  or 
3FeuS12+36H2O+8O=llFe3O4+36H2S+k. 

(6)  4FenSI2+33HJ0+1620=ll(2Fe2Os.3H20)+48S02+k.A 


ALTERATIONS  OF  PYRKHOTITE,  PYRITE,  AND  MARCASITE.     215 

If  iu  the  production  of  the  limonite  the  pyrrhotite  passes  through  the 
stage  of  ferrous  sulphate  the  reaction  producing  the  sulphate  may  be: 

(7)     Fe,,Si,+46O=llFeSO4+SO2+k,  or 
FeuS12+H2O+43O=llFeSO4+H.,S+k. 

The  change  from  the  ferrous  sulphate  to  the  limonite  is  the  same  as  in 
the  case  of  pyrite  and  marcasite.  Where  water  is  present  the  SO2  produced 
in  the  above  reactions  would  unite  with  water  and  form  H2SO3,  or  if  further 
oxidized  H2S04. 

As  the  end  results  of  alteration  are  usually  limonite  or  magnetite,  the 
volume  relations  for  these  two  compounds  will  be  given.  In  the  change 
of  pyrite  to  limonite  the  volume  is  increased  2.93  per  cent;  to  magnetite, 
is  decreased  37.48  per  cent.  In  the  change  from  marcasite  to  limonite  the 
volume  is  decreased  0.14  per  cent;  to  magnetite,  is  decreased  39.34  per 
per  cent.  In  the  change  of  pyrrhotite  to  magnetite  the  volume  is  decreased 
24.27  per  cent;  to  limonite,  is  increased  24.68  per  cent. 

When  pyrite  and  marcasite  pass  into  limouite  there  is  a  change  from 
a  crystalline  to  an  amorphous  form.  In  the  alteration  of  pyrite  to  magne- 
tite the  system  does  not  change.  In  the  alterations  of  pyrrhotite  and  mar- 
casite to  pyrite  there  are  changes  from  lower  degrees  of  symmetry  to  the 
highest  degree  of  symmetry,  that  of  the  isometric  system.  The  change 
from  marcasite  to  pyrite  occurs  especially  in  the  zone  of  anamorphism, 
subject  to  the  principle  there  obtaining  that  the  changes  take  place  with 
decrease  in  volume.  The  change  of  marcasite  to  pyrite  is  an  excellent 
illustration  of  the  principle  that  where  the  pressure  is  great  minerals  tend 
to  pass  into  other  minerals  having  a  higher  degree  of  symmetry  and  a  higher 
specific  gravity  (see  pp.  360-365).  The  abundance  of  marcasite  as  an 
autogenic  constituent  in  rocks  not  deeply  buried,  its  absence  in  the  rocks 
which  have  been  in  the  lower  zone,  and  the  presence  of  pyrite  in  these 
rocks,  are  thus  all  explained.  Where  the  pressure  is  small  near  the  surface 
marcasite  with  lower  symmetry  and  lower  specific  gravity  than  pyrite  may 
abundantly  form.  At  depth  where  the  pressure  is  great  pyrite  of  higher 
specific  gravity  and  higher  symmetry  forms.  If  rocks  near  the  surface  in 
which  marcasite  has  formed  are  buried  to  a  great  depth  by  superimposed 
strata  the  marcasite  previously  formed  changes  to  pyrite. 

Similar  statements  can  not  be  made  concerning  pyrrhotite  and  pyrite, 
for  these  minerals  have  unlike  compositions.  Doubtless  where  the  necessary 


216  A  TREATISE  ON  METAMORPHISM. 

chemical  reactions  can  take  place  there  is  a  tendency  in  the  lower  zone  for 
pyrrhotite  to  alter  to  pyrite. 

The  natural  conditions  for  the  transformation  of  pyrite,  marcasite,  and 
pyrrhotite  to  limonite  are  those  of  abundance  of  oxygen  and  moisture. 
These  conditions  are  found  in  the  zone  of  katamorphism,  and  especially  in 
the  belt  of  weathering.  In  this  belt  the  process  goes  on  with  such  rapidity 
that  pyrite,  marcasite,  and  pyrrhotite  have  generally  been  completely 
oxidized  where  the  rocks  have  been  long  exposed  to  the  reactions  of  the 
belt.  The  reactions  are  oxidation  and  hydration.  They  take  place  with 
great  liberation  of  heat  and,  for  pyrite  and  pyrrhotite,  with  some  expansion 
of  volume,  and  these  changes  may  therefore  be  taken  as  typical  illustrations 
of  alterations  of  the  belt  of  weathering. 

The  conditions  for  the  formation  of  magnetite  from  pyrite,  marcasite, 
and  pyrrhotite  are  the  presence  of  some  oxygen,  but  not  a  sufficient  amount 
to  fully  oxidize  the  iron,  and  considerable  pressure.  Where  iron  carbonate 
is  present,  which  also  alters  to  magnetite,  oxygen  is  not  necessary.  This 
reaction  is  of  great  consequence.  (See  p.  244.)  The  alterations  of  the 
sulphides  to  magnetite  involve  a  decrease  of  volume  of  24  to  39  per  cent 
and  liberation  of  heat.  Corresponding  with  this  fact,  the  changes  take 
place  in  the  belt  of  cementation  or  in  the  zone  of  anamorphism. 

THE    FLUORIDES. 

Among  the  fluorides  the  only  important  rock-making  mineral  is  fluorite. 

FLUORITE. 

Fluorite: 
CaF2. 
Isometric. 
Sp.  gr.  3.01-3.25. 

occurrence — Fluorite  occurs  as  an  accessory  constituent,  especially  in 
granitic  and  syenitic  rocks.  It  is  also  found  in  other  eruptive  rocks,  and 
in  metamorphic  rocks,  such  as  the  schists  and  marbles.  It  therefore  has  a 
somewhat  widespread  occurrence,  but  is  of  very  subordinate  importance. 

Alteration. — By  the  action  of  alkaline  waters  fluorite  alters  into  calcite 
(rhornbohedral;  sp.  gr.  2.7135).  Supposing  the  alkaline  compound  to  be 
sodium  carbonate,  the  reaction  is : 

CaF,+Na,CO,=CaCOs+2NaF+k. 

The  increase  in  volume  of  the  calcite  as  compared  with  the  fluorite  is  47.66 
per  cent. 


OCCURRENCE  OF  QUARTZ.  217 

THE  OXIDES. 

The  more  important  oxides  occurring  as  rock-building  constituents  are 
those  of  silicon,  iron,  and  titanium.  The  oxides  of  silicon  are  quartz, 
tridymite,  and  opal.  The  important  oxides  of  iron  are  hematite,  magnetite, 
and  limonite.  The  important  oxides  of  titanium  are  rutile,  octahedrite, 
and  brookite.  One  oxide  of  iron  and  titanium,  or  else  a  ferrous  titanate, 
has  a  widespread  occurrence;  this  is  ilmenite. 

QUARTZ. 

Quartz: 
SiO,. 

Rhombohedral. 
Sp.  gr.  2.653-2.654. 

occurrence. — Quartz  is  second  in  abundance  only  to  the  minerals  of  the 
feldspar  group.  According  to  Clarke,0  quartz  comprises  12  per  cent  of 
the  lithosphere.  It  is  very  abundant  as  an  original  pyrogenic  constituent 
of  the  igneous  rocks,  as  an  allogenic  constituent  of  the  clastic  rocks,  and 
as  an  autogenif,  mineral  in  all  classes  of  metamorphosed  rocks.  The 
material  for  secondary  quartz  may  be  derived  from  the  alterations  of  many 
minerals  in  situ,  or  from  the  decomposition  of  minerals  at  some  distance. 
The  most  widespread  of  all  the  alterations  which  furnish  silica  to  the 
solutions  is  that  of  the  decomposition  of  the  silicates  by  carbonic  acid  iu 
the  belt  of  weathering,  with  the  simultaneous  production  of  carbonates  and 
quartz,  or  a  solution  of  colloidal  silicic  acid  from  which  opal,  chert,  or 
quartz  may  later  separate.  Such  quartz  may  be  extensively  deposited  from 
the  solutions  in  the  porous  rocks  of  the  belt  of  cementation.  It  there  fills 
the  minute  spaces  between  the  individual  grains  of  sedimentary  rocks. 
It  occupies  spaces  iu  porous  tuffs  or  in  vesicular  igneous  rocks.  It  fills 
openings  between  laminae,  and  joint,  fault,  and  breccia  openings.  The 
quantity  of  quartz  thus  deposited  is  far  greater  than  that  of  any  other 
mineral,  and  not  improbably  greater  than  that  of  all  other  minerals  com- 
bined. By  this  process  the  rocks  are  cemented.  (See  pp.  617-621.)  Not 
only  may  the  openings  be  occupied  by  quartz,  but  at  the  time  of  the 
deposition  of  the  quartz  other  minerals  may  dissolve  and  their  places  be 
taken  by  the  quartz.  This  process  of  deposition  of  silica  as  quartz  is  called 
silicification.  (See  p.  205.) 

"Clarke,  F.  W.,  Analyses  of  rocks  from  the  laboratory  of  the  United  States  Geological  Survey, 
1880-1899:  Bull.  U.  S.  Geol.  Survey  No.  168,  1900,  p.  16. 


218  A  TREATISE  ON  METAMORPHISM. 

As  a  metamorphic  mineral,  quartz  is  derived  from  actinolite,  anorthite, 
anorthoclase,  anthophyllite,  augite,  biotite,  bronzite,  chalcedony,  cumming- 
tonite,  diopside,  enstatite,  epidote,  garnet,  grossularite,  hornblende,  hypers- 
thene,  microcline,  olivine,  opal,  orthoclase,  plagioclase,  prehnite,  pyrope, 
sahlite,  scapolites,  serpentine,  tridymite,  and  zoisite. 

Modifications. — The  most  frequent  and  important  modification  of  quartz  is 
by  recrystallization.  Crystallized  quartz  is  dissolved  under  conditions  of 
weathering,  as  are  all  other  minerals.  This  process  is,  however,  exceed- 
ingly slow.  As  a  result  of  solution  the  quartz  crystals  may  be  corroded. 
Such  corrosion  has  been  described  by  Hayes."  In  the  belt  of  cementation, 
and  especially  adjacent  to  trunk  channels  of  circulation,  quartz  may  be  ex- 
tensively dissolved  from  veins  and  from  the  wall  rocks.  (See  pp.  848-849.) 

Granulation  and  recrystallization  of  quartz  occur  on  a  most  extensive 
scale  in  all  quartzose  rocks  which  are  subjected  to  mass-mechanical  action 
or  other  favorable  conditions  in  the  zone  of  anamorphism.  These  changes 
involve  no  heat  and  volume  reactions  so  fat1  as  the  quartz  itself  is  concerned, 
except  that  as  the  original  minerals  may  be  strained,  or  the  new  grains  are 
imperfectly  adjusted,  the  change  may  involve  a  slight  expansion.  But 
such  expansion  is  followed  by  an  equal  contraction  when  the  material  is 
recrystallized  into  quartz  free  from  strain.  In  the  recrystallization  many 
small  individuals  may  be  merged  into  one  large  individual.  In  some 
instances  of  recrystallization,  where  large  grains  are  produced  from  smaller 
ones,  the  large  individuals  may  average  more  than  a  million  times  as  great 
as  the  small  individuals  from  which  they  are  derived.  (See  p.  695.)  In 
the  production  of  a  comparatively  few  large  individuals  from  a  multi- 
tude of  small  individuals  there  is  probably  a  release  of  energy.  (See 
p.  771.)  During  recrystallization  the  material  taken  into  solution  may 
be  deposited  practically  in  situ  or  may  travel  far  and  be  extensively 
deposited  elsewhere.  Often  quartz  deposited  in  situ,  or  nearly  so,  can  not 
be  discriminated  from  quartz  deposited  from  solutions  coming  from  distant 
sources,  as  above  described. 

A  second  modification  of  quartz  only  less  important  than  that  of 
recrystallization  is  silication  by  the  union  with  bases  united  with  other 
acids,  thus  forming  silicates.  Of  such  acids  carbonic  is  by  far  of 

a  Hayes,  C.  W.,  Solution  of  silica  under  atmospheric  conditions:  Bull.  Geol.  Soc.  America,  vol. 
8,  1897,  pp.  213-220. 


MODIFICATIONS  OF  QUARTZ.  219 

the  greatest  consequence;.  Some  of  the  more  common  minerals  in  which 
silication  occurs  on  an  extensive  scale  are  calcite,  dolomite,  ankerite,  and 
siderite,  thus  producing  wollastonite,  diopside,  tremolite,  sahlite,  actinolite, 
and  griinerite.  The  silica  ma}'  unite  with  the  bases  of  various  carbonates 
producing  various  complex  silicates,  such  as  chondrodite,  augite,  horn- 
blende, garnet,  etc.  At  the  same  time  the  material  of  previous  silicates 
may  be  absorbed.  The  heat  and  volume  reactions  in  many  of  these 
changes  may  be  found  under  the  carbonates  mentioned. 

In  this  process  of  silication  of  carbonates  it  is  not  often  possible  to 
identify  the  remnants  of  the  quartz  individuals  which  furnished  the  silica 
for  the  reactions.  But  apparently  the  quartz  particles  which  furnished  the 
silica  for  the  process  of  silication  may  be  identified  in  some  instances. 
This  is  best  seen  for  such  fibrous  minerals  as  serpentine,  talc,  and  actinolife, 
the  needles  or  fibers  of  which  appear  to  grow  into  the  quartz,  in  some 
instances  deeply.  In  such  cases  it  seems  clear  that  the  silica  of  the  quartz 
furnished  at  least  a  part  of  the  silica  for  the  silicate,  the  bases  being 
furnished  by  the  solutions. 

One  of  the  best  instances  of  the  extensive  union  of  quartz  with  bases, 
producing  serpentine  pseudomorphous  after  quartz,  is  that  described  by 
Becker."  He  describes  the  exteriors  of  original  clastic  grains  of  quartz  to 
be  "entirely  occupied  by  felted  fibers  of  serpentine,  and  long,  slender 
microlites  pierce  the  quartz  grain  toward  its  center,  like  pins  in  a  cushion.'"1 
This  is  but  one  illustration  of  a  very  widespread  replacement  of  quartz 
by  serpentine  in  the  Coast  Ranges.  The  growth  of  actinolite  into  quartz 
is  illustrated  in  the  Tyler  slate  of  the  Penokee  district  of  Wisconsin."0 

In  instances  where  the  quartz  furnishes  the  silica  for  the  penetrating 
silicates  the  migration  of  the  silica  is  microscopical,  and  it  might  be  sup- 
posed that  the  reactions  occur  without  the  solution  of  the  silica  of  the 
quartz;  but  it  seems  probable,  even  in  such  cases  as  these,  that  there  is 
solution  of  the  silica  before  combination  with  the  bases.  In  such  reactions 
it  is  presumed  that  the  bases  which  unite  with  the  silica  were  before  united 
with  some  other  acid,  and  it  is  only  when  the  previous  combination  is  known 
that  the  heat  and  volume  relations  of  the  reactions  can  be  ascertained. 

"Becker,  G.  F.,  Geology  of  the  quicksilver  deposits  of  the  Pacific  slope:  Mon.  U.  8.  Geol. 
Survey,  vol.  13,  1888,  pp.  120-127. 

6  Becker,  cit.,  p.  124. 

« Irving,  R.  D.,  and  Van  Hise,  C.  R.,  The  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin: 
Mon.  U.  S.  Geol.  Survey,  vol.  19,  1892,  pp.  210-215. 


220  A  TREATISE  ON  METAMORFHISM. 

In  a  third  class  of  changes  quartz  may  be  wholly  replaced  by  other 
minerals,  as  by  magnetite  and  hematite.  Very  frequently  the  deposition  of 
the  new  minerals  seems  to  be  conditioned  upon  the  solution  of  the  quart/. 
The  replacement  of  quartz  by  iron  oxide  is  illustrated  in  the  Lake  Superior 
region  in  both  the  iron-bearing  and  the  slate  formations." 

The  most  favorable  conditions  for  the  solution  of  silica,  especially  of  that 
formed  by  the  decomposition  of  the  silicates  by  carbonation,  are  furnished 
by  the  belt  of  weathering.  The  most  favorable  conditions  for  the  deposi- 
tion of  silica  as  quartz  are  those  of  the  belt  of  cementation.  The  solution  of 
silica  in  the  belt  of  weathering  of  the  zone  of  katamorphism  and  its  deposi- 
tion in  the  belt  of  cementation  of  this  zone  is  perhaps  the  best  illustration 
of  the  principle  explained  on  pages  634-636,  that  material  dissolved  in 
the  belt  of  weathering  may  be  extensively  deposited  in  the  belt  of 
cementation.  Recrystallization  of  quartz  mainly  takes  place  in  the  zone 
of  anamorphism,  although  it  undoubtedly  occurs  to  some  extent  in  the 
zone  of  katamorphism,  and  especially  in  the  belt  of  cementation.  The 
process  of  silication  takes  place  almost  invariably  with  decrease  in  volume, 
provided  all  the  compounds  concerned  are  solids.  Where  the  carbonates 
are  silicated  the  decrease  in  volume  ranges  from  20  to  40  per  cent.  Silica- 
tion occurs  upon  a  great  scale  in  the  zone  of  anamorphism — is,  indeed,  one 
of  the  most  distinctive  chemical  reactions  of  that  zone. 

TRIDYMITE. 

Tridymite: 
SiO2 

Hexagonal,  or  pseudo-hexagonal. 
Sp.gr.  2.28-2.33. 

occurrence. — Tridymite  usually  occurs  as  an  autogenic  mineral  in  cavities 
in  lavas,  such  as  rhyolite,  andesite,  trachyte,  etc. 

Modifications. — Tridymite  is  dissolved  more  readily  than  quartz.  The 
material  of  tridymite  may  go  through  any  of  the  changes  which  silica  of 
quartz  may  pass  through,  with  the  difference  that  its  recrystallization  would 
result  in  the  production  of  quartz  (rhombohedral ;  sp.  gr.  2.652-2.654) 
rather  than  the  original  mineral,  tridymite.  The  changes  of  tridymite  into 
other  minerals  than  quartz  need  not  be  discussed  in  detail,  since  the  reac- 
tions are  the  same  as  with  quartz,  except  that  the  volume  decrease  is  greater 

«Van  Hise,  C.  R.,  and  Bayley,  W.  S.,  The  Marquette  iron-bearing  district  of  Michigan:  Mon.  U.  S. 
Geol.  Survey,,  vol.  28,  1897,  pp.  370,  400-405. 


OPAL.  221 

in  the  changes  of  tridyrnite  than  with  quartz.  In  the  change  of  tridymite 
to  quartz  there  is  a  diminution  of  volume,  amounting  to  14.24  per  cent,  and 
there  is  also  probably  liberation  of  heat.  Energy  is  therefore  poteutialized 
in  tridymite  as  compared  with  quartz.  The  change  is  one  which  is  particu- 
larly likely  to  occur  in  the  zone  of  anamorphism,  where  pressure  is  the 
dominant  factor.  In  the  fact  that  quartz  is  a  denser  mineral  than 
tridymite  we  probably  have  a  reason  not  only  for  the  passage  of  tridymite 
into  quartz  in  the  lower  zone,  but  for  the  absence  of  tridymite  as  an 
original  pyrogenic  constituent  in  the  plutonic  igneous  rocks  which  crystal- 
lized originally  in  this  zone.  Under  its  conditions  the  denser  mineral, 
quartz,  formed. 

OPAL. 

Opal: 

SiO2.nH2O     ( H2O  2  to  13  per  cent;  but  mostly  3  to  9  per  cent. ) 

Amorphous. 

Sp.gr.  2.1-2.2. 

occurrence. — Opal,  like  most  other  hydrous  minerals,  is  a  product  of  the 
zone  of  katamorphism.  Opal  is  a  direct  deposit  from  hot  springs.  In  the 
sedimentary  rocks  it  is  abundantly  formed  from  the  siliceous  skeletons  of 
certain  animals  and  plants,  such  as  radiolaria,  sponges,  and  diatoms.  Opal 
is  plentifully  deposited  in  cavities  in  rocks  by  subterranean  waters.  Its 
most  common  places  of  occurrence  are  the  limestones,  where  it  is  largely 
of  organic  origin,  and  the  porous  igneous  rocks,  especially  as  amygdules  of 
the  amygdaloids,  where  it  is  a  chemical  precipitate. 

In  general,  as  a  metamorphic  product  opal  may  be  derived  from  the 
same  minerals  as  quartz. 

Modifications. — The  most  frequent  change  of  opal  is  to  quartz  (rhombohe- 
dral;  sp.  gr.  2.652-2.654).  Frequent  intermediate  products  are  chalcedony 
and  chert,  which  appear  to  be  partly  crystalline  substances.  (See  p.  222.) 
In  the  passage  of  opal  into  quartz,  the  changes  are  three:  dehydration, 
reduction  of  volume,  and  recrystallization.  Supposing  the  composition  of 
the  opal  is  SiO2.|H2O,  which  would  be  about  6  per  cent  of  water,  the 
decrease  of  volume  would  be  22.81  per  cent.  The  change  from  opal  to 
quartz  above  given  is  commonly  accomplished  by  solution  and  redeposition 
or  recrystallization.  When  the  material  is  taken  into  solution  this  silica  may 
be  deposited  near  by  or  transported  elsewhere.  It  may  unite  with  free 
bases,  producing  silicates;  it  may  displace  other  acids  combined  with  bases, 


222  '  A  TREATISE  ON  METAMORPHISM. 

as,  for  instance,  carbonic  acid,  thus  also  producing  silicates.     The  heat  and 
volume  relations  of  these  reactions  are  discussed  under  "Quartz." 

The  reactions  of  dehydration,  crystallization,  and  lessening  of  volume, 
as  seen  on  pages  167-170,  are  particularly  characteristic  of  the  zone  of 
anamorphism,  and  it  is  in  this  zone  that  the  change  from  opal  to  quartz 
probably  most  extensively  occurs.  As  evidence  of  this  is  the  frequent 
occurrence  of  opal  in  the  zone  of  katamorphism,  and  the  general  absence 
of  opal  in  the  rocks  which  have  been  metamorphosed  in  the  lower  zone. 

CHERT,    CHALCEDONY,  ETC. 

Standing  between  opal  and  quartz  are  numerous  varieties  of  partly 
crystallized  or  very  finely  crystallized  silica,  of  which  chert  and  chalced- 
ony may  be  taken  as  the  more  important  kinds.  With  these  substances 
are  frequently  small  but  variable  amounts  of  opal  containing  combined 
water.  The  specific  gravities  of  chert  and  chalcedony  are  intermediate 
between  those  of  opal  and  quartz,  i.  e.,  between  2.15  and  2.65.  Their  most 
frequent  occurrence  is  as  veins,  nodules,  belts,  and  members  in  carbonate 
formations.  Ordinarily  they  are  derived  from  organic  forms,  such  as 
radiolaria,  diatoms,  and  sponges,  which  lived  under  conditions  similar  to 
those  under  which  the  limestone-building  animals  lived.  (See  p.  817.) 

Chert  and  chalcedony  are  derived  from  opal.  The  material  here 
included  varies  from  that  which  is  close  to  opal,  having  only  a  few  minute 
crystallized  spots,  through  material  which  shows  more  and  more  evidence  of 
crystallization,  to  material  which  contains  comparatively  little  amorphous 
silica,  and  thence  into  fully  crystallized  silica  or  quartz.  The  transition 
varieties  may  have  the  peculiar  spotty  appearance  in  polarized  light  char- 
acteristic of  ordinary  chert  or  the  peculiar  radial  fibrous  polarization  of 
chalcedony  or  any  combination  of  the  two. 

The  alterations  of  chert  and  chalcedony  are  into  quartz,  or  by  combi- 
nation with  bases  producing  silicates,  the  same  as  opal.  The  chemistry  and 
physics  of  the  change  are  the  same  as  for  opal  except  that  the  decrease  in 
volume  is  less,  and  therefore  they  need  not  be  repeated. 


OCCURRENCE  OF  CORUNDUM.  223 

,     HEMATITE    GROUP. 
CORUNDUM,  HEMATITE,  AND  ILHEMTE. 

Corundum: 

A12O3. 

Rhonibohedral. 

Sp.  gr.  3  95-1.10. 
Hematite: 

Fe.A- 

Rhombohedral. 
Sp.  gr.  5.20-5.25. 
Ilmenite: 

FeTiOs;  varies  to  mFeTiOs  .nFe.;O,. 

Rhombohedral. 

Sp.  gr.  4.50-5.02. 

CCRUNDUM. 

occurrence. — In  Canada  at  one  locality  corundum  occurs  as  an  original 
constituent  of  a  syenite."  Also,  corundum  as  an  accessory  mineral  has  been 
noted  in  granite,  andesite,  and  other  rocks.  Corundum  is,  therefore,  an 
original  pyrogenic  constituent  of  igneous  rocks.  Corundum  occurs  along 
the  contact  of  intrusive  basic  rocks  rich  in  alumina,  especially  those  con- 
taining more  than  30  per  cent,  such  as  peridotites  and  pyroxenites.  The 
intruded  rocks  may  be  either  igneous  rocks  or  gneisses  and  schists.  But 
where  corundum  occurs  in  veins  along  contacts  it  is  in  many  cases  an 
aqueo-igneous  product  (see  pp.  720-728)  or  an  aqueous  deposit.  Corundum 
is  a  widespread  accessory  constituent  in  various  micaceous,  chloritic,  and 
hornblendic  schists  and  gneisses,  and  in  marble.  Corundum,  as  a  meta- 
morphic  mineral,  is  associated  with  chlorite  and  corundophilite.  It  is  often 
associated  with  other  heavy  metamorphic  minerals,  such  as  andalusite, 
sillimanite,  cyanite,  spinel,  rutile,  etc.  As  a  metamorphic  mineral  it  is 
derived  from  andalusite,  cyanite,  diaspore,  gibbsite,  sillimanite,  staurolite, 
and  topaz. 

Alterations. — Corundum  alters  into  diaspore  (orthorhombic;  sp.  gr.  3.40), 
gibbsite  (monoclinic ;  sp.  gr.  2.35),  spinel  (isometric;  sp.  gr.  3  8),  sillimanite 
(orthorhombic;  sp.  gr.  3.235),  cyanite  (triclinic;  sp.  gr.  3.615),  muscovite 
(damourite),  (monoclinic;  sp.  gr.  2.88),  margarite  (monoclinic;  sp.  gr.  3.035), 
and  zoisite  (orthorhombic;  sp.  gr.  3.31).  The  reactions  for  the  formation  of 
diaspore  and  gibbsite  are  simple  reactions  of  hydration.  The  reactions  for 

"Miller,  VV.  G.,  Economic  geology  of  eastern  Ontario;  corundum  and  other  minerals:   Seventh 
Kept.  Ontario  Bureau  of  Mines,  1897,  Toronto,  1898,  p.  213. 


224  A  TREATISE  ON  METAMORPHISM. 

the  production  of  the  other  minerals  require  the  addition  of  various  other 
constituents — in  the  case  of  spinel,  magnesia ;  in  the  case  of  sillimanite  and 
cyanite,  silica;  in  the  case  of  the  complicated  silicates,  muscovite,  margarite, 
and  zoisite,  various  bases  and  a  large  amount  of  silica.  Therefore  in  these 
cases  it  is  clear  that  the  common  statement  that  corundum  alters  to  the 
minerals  muscovite,  margarite,  and  zoisite  can  have  only  the  meaning  that 
the  relations  are  such  that  corundum  furnishes  the  alumina  for  the  resultant 
compound,  and  that  the  additional  compounds  are  derived  from  another 
source.  It  will  be  assumed  in  the  alterations  that  the  magnesia,  lime, 
and  potash  are  derived  from  the  solid  carbonates  and  that  the  siliga  is 
added  as  quartz.  The  equations  for  the  reactions  are  as  follows: 

(1)  Al2O,+HsO=2[AlO.(OH)]+k. 

(2)  Al,Os-i-3H20=2Al(OH)s+k. 

(3)  Al20,+MgCOs=MgAlA+C02+k. 

(4)  AlA+SiO2=Al2SiO6+k. 

(5)  3Al203+6SiO2  +  K2CO3+2H2O=2HsKAl3SisO,2+CO2+k. 

(6)  2AlA+2Si02+CaC03+HA=H2CaAl4Si2012+CO,+k. 

(7)  3Al2Os+6SiO2+4CaCO,+H2O=H2Ca,Al6Si6O26+4CO2+k. 

The  increase  in  volume  as  compared  with  corundum  is,  for  diaspore 
(equation  1),  39.25  per  cent;  for  gibbsite  (equation  2),  161.83  per  cent. 
The  volume  of  the  corundum  and  the  magnesite  in  passing  to  the  spinel 
(equation  3)  is  decreased  29.17  per  cent.  The  volume  of  the  corundum 
and  quartz  in  passing  into  sillimanite  (equation  4)  is  increased  4.38  per 
cent;  into  cyanite  (equation  4)  is  decreased  6.59  per  cent.  If  the  volume 
of  the  corundum  be  compared  with  that  of  the  muscovite  (equation  5), 
with  that  of  the  margarite  (equation  6),  and  with  that  of  the  zoisite 
(equation  7),  there  will  be  great  volume  increases.  If,  on  the  other 
hand,  all  the  products  which  unite  with  the  corundum  in  each  case,  with 
the  exception  of  the  water,  be  counted  as  solid,  there  would  be  small 
inci'ease  in  the  volume  for  muscovite,  a  considerable  decrease  for  zoisite, 
and  a  small  decrease  for  margarite.  On  the  first  hypothesis  the  increase  in 
the  volume  in  the  production  of  muscovite  is  264.25  per  cent;  in  margarite, 
159.02  per  cent;  in  zoisite,  261.34  per  cent.  On  the  second  hypothesis  the 
increase  in  volume  in  the  production  of  muscovite  is  1.62  per  cent;  to  form 
margarite  the  decrease  is  1.22  per  cent;  to  form  zoisite  the  decrease  is  23.58 
per  cent. 

It  is  reasonably  certain  that  the  passage  of  corundum  to  diaspore  and 
gibbsite  is  a  reaction  characteristic  of  the  zone  of  katamorphis-m,  and 


OCCURRENCE  OF  HEMATITE.  225 

especially  the  belt  of  weathering.  It  is  almost  equally  certain  that  the 
passage  of  corundum  into  spinel,  sillimanite,  and  cyanite  is  characteristic 
of  the  zone  of  anamorphism. 

The  case,  however,  is  not  clear  in  reference  to  the  muscovite,  margarite, 
aiid  zoisite.  The  equations  as  written  are  those  of  silicifiation  and  slight 
hydration.  If  these  equations  be  correct,  they  should  occur  in  the  lower 
part  of  the  belt  of  cementation  or  in  the  zone  of  anamorphism.  It  is 
tolerably  certain  that  margarite,  zoisite,  and  muscovite  form  in  the  lower 
part  of  the  belt  of  cementation;  but  the  zone  in  which  muscovite  charac- 
teristically develops  is  that  of  anamorphism.  It  is  not  at  all  impossible 
that  the  potassium  carbonate,  and  perhaps  the  calcium  carbonate,  or  even 
the  silica,  are  added  in  solution  for  the  margarite  and  zoisite.  In  this  case 
there  would  be  a  considerable  volume  increase.  Whether  the  same  may 
be  assumed  for  the  muscovite  is  uncertain.  Very  likely  the  materials 
added  to  the  corundum  are  in  some  cases  carried  in  by  the  solutions,  in 
others  are  derived  from  adjacent  minerals,  and  in  still  others  partly  from 
both.  Where  the  lime  and  potash  are  derived  from  minerals  adjacent,  they 
may  come  from  other  compounds  than  carbonates,  and  the  silica  may  have 
been  previously  united  with  other  bases.  So  far  as  this  is  so,  in  considering 
the  variations  in  volume  the  minerals  from  which  the  elements  added  to  the 
corundum  to  produce  the  muscovite,  margarite,  and  zoisite  were  derived 
must  be  taken  into  account.  It  is  clearly  impracticable  in  the  present  state 
of  knowledge  to  give  definite  statements  as  to  the  volume  changes  for  these 
minerals. 


HEMATITE. 


occurrence. — Hematite  is  a  pyrogenic  constituent  in  igneous  rocks  and  is 
an  abundant  metamorphic  mineral.  Its  most  abundant  source  in  the 
metamorphic  rocks  is  by  the  dehydration  of  limonite,  a  reaction  occurring 
with  the  absorption  of  heat  and  reduction  of  volume.  A  second  important 
source  of  hematite  is  from  iron  carbonate  by  loss  of  carbon  dioxide  and  by 
oxidation,  a  reaction  occurring  with  the  liberation  of  heat  and  reduction  of 
volume.  Hematite  may  also  be  produced  by  the  oxidation  of  magnetite,  a 
reaction  resulting  in  liberation  of  heat  and  expansion  of  volume.  Fre- 
quently after  this  change  the  hematite  has  the  isometric  form  of  the  original 
magnetite  and  is  called  martite.  A  fourth  source  of  hematite  is  by  the 
oxidation  of  the  ferrous  iron  of  silicates  at  the  time  of  their  decomposition. 
MON  XLVII — 04 15 


926  A  TREATISE  ON  METAMORPHISM. 

A  fifth  source  is  by  oxidation  of  ferrous  iron  solutions,  which  may  result  in 
the  precipitation  of  hematite.  The  first  reaction  occurs  most  extensively 
in  the  zone  of  anamorphism;  the  other  four  occur  in  the  zone  of  katamor- 
phism,  and  to  these  positions  the  heat  and  volume  reactions  correspond. 
Finally,  as  shown  in  Chapter  XII,  on  "Ore  deposits,"  hematite  may  be 
formed  from  pyrite  by  the  action  of  alkaline  carbonate  solutions. 

In  summary,  hematite  is  derived  from  actinolite,  ankerite,  anthophyl- 
lite,  biotite,  bronzite,  garnet,  greenalite,  grunerite,  hornblende,  hypers- 
thene,  ilmenite,  limonite,  magnetite,  olivine,  parankerite,  pyrite,  serpentine, 

and  siderite. 

Alteration.. — The  most  frequent  alteration  of  hematite  is  into  limonite 
(amorphous;  sp.  gr.  3.6-4).  The  reaction  is  as  follows: 

2Fe,03+3H20=2FeA.3HirO+k. 

Iii  the  change  the  volume  is  increased  60.72  per  cent.  A  second  altera- 
tion of  hematite  is  into  magnetite  (isometric;  sp.  gr.  5.168-5.18).  This 
may  be  accomplished  by  any  of  the  reducing  agents  furnished  by  organic 
compounds.  Supposing  the  reducing  agent  to  be  the  partially  oxidized 
carbon  compound  CO,  the  reaction  is: 

3Fe2Os+CO=2Fe,O4+CO2+k. 

While  a  reduction  of  the  oxide  of  iron  occurs  a  simultaneous  oxidation  of 
the  organic  compound  occurs,  and  the  end  result  is  the  liberation  of  heat. 
In  the  change  the  volume  is  decreased  2.38  per  cent.  A  third  alteration 
of  hematite  is  to  pyrite  (isometric;  sp.  gr.  5.025)  or  marcasite  (orthorhom- 
bic;  sp.  gr.  4.875).  In  the  best-known  instances  siderite  (rhombohedral ; 
sp.  gr.  3.855)  or  some  other  iron-bearing  carbonate  is  simultaneously 
produced.  The  reaction  may  be: 

Fe,0,+2H2S+CO2=FeS2+FeCOs+2H,0+k. 

In  the  change  to  pyrite  and  siderite  the  volume  is  increased  76.12  per  cent, 
and  to  marcasite  and  siderite  78.73  per  cent. 

The  alterations  of  hematite  to  limonite  occur  in  the  zone  of  katamor- 
phism,  and  especially  in  the  belt  of  weathering.  Corresponding  with  this 
position  the  reaction  is  with  liberation  of  heat  and  expansion  of  volume. 
The  alteration  of  hematite  into  magnetite  occurs  in  the  belt  of  cementation 
and  the  zone  of  anamorphism.  This  agrees  with  the  fact  that  the  reaction 


ALTERATION  OF  ILMENITE.  227 

liberates  heat  and  diminishes  the  volume.  The  alteration  of  hematite  to 
pyrite  and  marcasite  is  best  known  where  organic  compounds  are  present 
to  reduce  sulphuric  acid  to  hydrosulphuric  acid  and  to  furnish  carbonic  acid 
to  form  the  carbonates.  The  reaction  is  especially  characteristic  of  the  belt 
of  cementation,  and  to  this  position  the  expansion  of  volume  and  the 
liberation  of  heat  correspond. 

ILMENITE. 

occurrence. — Ilmenite  is  an  abundant  pyrogenic  constituent  of  the  igneous 
rocks.  It  is  found  both  as  an  allogenic  and  as  an  autogenic  constituent  in 
metamorphic  rocks.  As  an  autogenic  constituent  the  compounds  which 
unite  to  produce  it  have  not  been  worked  out.  As  a  metamorphic  mineral 
ilmeuite  is  derived  from  perovskite  and  rutile. 

Alterations. — Ilmeiiite  alters  to  titanite  (monoclinic;  sp.  gr.  3.48),  to  rutile 
(tetragonal;  sp.  gr.  4.18-4.25),  and  to  octahedrite,  or  anatase  (tetragonal; 
sp.  gr.  3.82-3.95).  With  these  minerals  magnetite  (isometric;  sp.  gr.  5.174) 
or  hematite  (rhombohedral;  sp.  gr.  5.225)  or  limonite  (amorphous;  sp. 
gr.  3. HO)  is  simultaneously  produced.  One  of  the  most  frequent  reactions 
in  the  production  of  titanite  is  probably  along  the  following  lines : 

3FeTiOs+3CaCO,+3SiO2+O=3CaTiSiO!i+Fe8O4+3COj+k. 

The  decrease  in  volume  of  the  ilmenite,  calcite,  and  quartz  in  passing  into 
titanite  and  magnetite,  supposing  the  CO2  to  escape,  is  22.35  per  cent;  but 
the  increase  in  volume  of  the  titanite  as  compared  with  the  ilmenite  alone 
is  76.35  per  cent.  The  alteration  of  ilraenite  to  rutile  and  octahedrite,  with 
combined  magnetite,  is  as  follows: 

3FeTiO3+O=3TiO2+Fes04+k. 

In  case  hematite  is  produced  instead  of  magnetite  the  reaction  is: 

2FeTiOs+O=2TiOj+Fe2O,,+k. 

In  case  limonite  is  produced,  one  and  one-half  molecules  of  water  are  added 
to  both  sides  of  the  equation. 

The  increase  in  volume  of  the  ilmenite  in  passing  into  rutile  and  mag- 
netite is  6.02  per  cent;  into  octahedrite  and  magnetite,  11.07  per  cent.  In 
case  hematite  or  limonite  be  produced,  the  increase  in  volume  is  corre- 
spondingly greater. 

It  is  certain  that  titanite  forms  from  ilmenite  in  the  lower  zone.  In  this 
zone,  as  explained  on  pp.  764-765,  the  CaCO3  and  SiO2  can  not  be  supposed 


228  A  TREATISE  ON  METAMORPHISM. 

to  have  been  brought  in  from  the  outside,  and  therefore  the  change  takes 
place  with  decrease  in  volume.  It  is  also  certain  that  titanite  forms  exten- 
sively in  connection  with  chlorite,  which  commonly  develops  in  the  belt  of 
cementation.  In  this  case  the  calcium  carbonate  and  silica  may  be  intro- 
duced in  solution  from  an  outside  source,  under  which  circumstances  the 
volume  is  increased. 

The  alterations  of  ilmenite  to  rutile  and  octahedrite,  or  any  combina- 
tion of  them,  certainly  occur  in  the  zone  of  katamorphism,  and  to  this 
position  the  heat  and  volume  reactions  correspond.  However,  I  have  not 
found  sufficient  information  on  the  subject  to  assert  that  these  reactions  do 
not  also  occur  in  the  zone  of  anamorphism. 

SPINEL   GROUP. 

SPINEL,  >U(.M,  I ;  I  I  .  AXD  i  II  IJOMI  1 1  . 
Spinel: 

MgALA- 

(Hercynite,  FeAl3O4-) 
(Pleonaste,  [MgFe]  [AlFe]204.) 
(Picotite,  [MgFe]  [AlCr]2O4-) 
Isometric. 
Sp.  gr.  3.5-4.1. 
Magnetite: 
Fe,04. 
Isometric. 
Sp.  gr.  5.168-5.180. 
Chromite: 

FeCr204- 

Isometric. 

Sp.  gr.  4.32-4.57. 

SPINEL. 

occurrence. — Spinel  occurs  as  an  original  constituent  in  the  igneous  rocks, 
but  is  much  more  abundantly  present  as  a  secondary  constituent  in  the 
metamorphic  rocks,  especially  those  which  are  rich  in  magnesium.  In 
many  cases  it  is  secondary  to  olivine  and  other  minerals  rich  in  magnesium. 
The  more  important  minerals  from  which  spinel  is  derived  are  almaiidite, 
biotite,  chlorite,  corundum,  diaspore,  garnet,  gibbsite,  olivine,  and  pyrope. 

Alterations. — According  to  Dana,  spinel  has  been  observed  as  altering  to 
talc  (orthorhombic  or  monoclinic;  sp.  gr.  2.75),  serpentine  (monoclinic; 
sp.  gr.  2.575),  and  mica  (mouoclinic;  sp.  gr.  2.88-2.90).  However,  the 
character  of  the  alterations  and  the  conditions  under  which  they  occur  are 
so  little  known  that  I  shall  not  attempt  to  treat  them  from  the  physical- 
•chemical  point  of  view. 


SPINEL  GROUP.  229 


MAGNETITE. 


occurrence. — Magnetite  is  a  very  abundant  pyrogenic  constituent  in 
igneous  rocks.  It  is  abundantly  deposited  from  solutions,  and  especially 
from  solutions  bearing  iron  carbonate,  according  to  the  reaction: 

3FeC03+O =FeA+ 3CO2 + k . 

Magnetite  also  extensively  forms  from  siderite  in  situ.  These  changes 
liberate  heat  and  decrease  the  volume.  A  third  source  of  magnetite  is  by 
incomplete  oxidation  of  pyrite  and  marcasite,  reactions  occurring  with 
liberation  of  heat  and  diminution  of  volume.  Fourth,  frequently  siderite 
and  iron  sulphide  together  pass  into  magnetite  with  decrease  in  volume. 
(See  pp.  244,  845.)  A  fifth  way  in  which  magnetite  may  be  produced  is 
by  the  reduction  of  hematite  by  organic  compounds,  a  reaction  occurring 
with  the  liberation  of  heat,  because  of  the  simultaneous  oxidation  of  the 
organic  compounds,  and  with  diminution  of  volume.  A  sixth  way  in  which 
magnetite  is  produced  is  by  the  incomplete  oxidation  of  ferrous  iron  of 
silicates;  for  instance,  olivine  and  garnet. 

In  summary,  magnetite  is  derived  from  actinolite,  ankerite,  arfvedsonite, 
augite,  biotite,  bronzite,  diopside,  garnet,  greenalite,  griinerite,  hematite, 
hornblende,  hypersthene,  ilmenite,  marcasite,  and  pyrite. 

Alterations. — Magnetite  alters  into  hematite  (rhombohedral;  sp.  gr.  5.225), 
lirnonite  (amorphous;  sp.  gr.  3.80),  and  siderite  (rhombohedral;  sp.  gr.  3.83- 
3.88).  The  reactions  are  as  follows: 

(1)  2Fe,O4+O=3Fe,0,+k. 

(2)  4Fe3O4+2O+9H2O=3(2Fe2Os-3H20)  +k. 

(3)  Fe3O4+CO+2CO2=3FeCO3+k. 

i 

In  the  change  the  increase  in  volume  is,  for  (1),  2.44  per  cent;  for  (2), 
64.63  per  cent,  and  for  (3),  101.30  per  cent.  The -increase  in  volume  in  the 
change  from  magnetite  to  siderite — over  100  per  cent — is  the  greatest 
volume  change  in  which  only  two  minerals  are  concerned  which  the  calcu- 
lations of  Mr.  Lincoln  have  given,  with  the  exception  of  the  alteration  of 
corundum  into  gibbsite.  (See  p.  224.)  All  of  the  above  changes  are  well 
known  to  occur  in  the  zone  of  katamorphism,  and  corresponding  with  this 
position  they  all  take  place  with  the  liberation  of  heat,  expansion  of  volume, 
and  decrease  in  symmetry. 

CHROMITB. 

occurrence. — Chromite  occurs  in  the  igneous  rocks,  especially  those  rich 
in  magnesium.  It  also  occurs  in  the  metamorphic  rocks,  often  in  connec- 


230  A  TREATISE  ON  METAMORPHISM. 

tion  with  serpentine.     In  these  positions  it  is  very  frequently  a  secondary 
product  of  olivine.     The  reactions  occurring  in  its  production  are  given 

under  that  mineral. 

Alterations. — The  alteration  of  chromite  into  other  minerals  has  not  been 

noted. 

RUTILE    GROUP. 

Ill  I  I  I.I  .   OCTAHEDBITE,  A>'D  BKOOKITE. 
Rutile: 

TiO2. 

Tetragonal. 

Sp.  gr.  4.18-4.25. 
Octahedrite: 

TiOv. 

Tetragonal. 

Sp.  gr.  3.82-3.95. 
Brookili:- 

TiO2. 

Orthorhombic. 

Sp.  gr.  3.87-1.082. 

occurrence. — Rutile  is  a  pyrogenic  constituent  in  igneous  rocks,  and  has  a 
widespread  occurrence  in  the  clastic  and  metamorphic  rocks,  both  as  an 
allogeuic  and  as  an  autogenic  constituent,  in  the  latter  case  generally  being 
derived  from  ilmenite.  Rutile  is  also  derived  from  brookite,  ilmenite,  octa- 
hedrite,  and  titanite. 

Octahedrite  in  the  metamorphic  rocks  is  a  secondary  alteration  of 
other  titanium-bearing  minerals,  especially  of  titanite.  It  is  also  derived 
from  ilmenite. 

Brookite  occurs  sparingly,  both  in  altered  igneous  rocks  and  in  sedi- 
mentary rocks.  In  some  cases  it  is  secondary  to  titanite. 

Alteration.. — Both  Octahedrite  and  brookite  alter  to  rutile  (tetragonal;  sp. 
gr.  4.18-4.25).  In  the  case  of  octahedrite  the  decrease  in  volume  is  7.83  per 
cent;  in  the  case  of  brookite  the  decrease  is  5.69  per  cent.  The  heat  change 
is  undetermined,  but  probably  the  alterations  occur  with  the  liberation  of 
heat.  If  this  be  the  case  the  alterations  involve  recrystallizatiou,  diminu- 
tion of  volume,  and  liberation  of  heat.  In  the  case  of  octahedrite  the  sym- 
metry remains  the  same;  in  the  case  of  brookite  the  symmetry  is  increased. 

It  may  be  inferred  that  such  changes  as  these  occur  in  both  zones, 
being  in  all  respects  analogous  to  the  changes  which  take  place  in  dolomiti- 
zation.  (See  pp.  238-240.)  However,  the  geological  occurrences  of  these 
alterations  have  not  been  given  with  such  definiteness  as  to  enable  one  to 
make  definite  statements  as  to  the  actual  facts. 


ALTERATIONS  OF  RUTILE.  231 

In  this  connection  the  experiments  of  Hautefeuille"  are  very  interesting. 
He  produced  rutile,  brookite,  and  octahedrite  from  the  same  compounds, 
but  at  different  temperatures,  rutile  forming  when  red  heat  was  used, 
brookite  when  the  temperature  was  between  that  required  for  the  volatiliza- 
tion of  cadmium  and  zinc,  and  octahedrite  when  the  temperature  was  a  little 
below  that  for  the  volatilization  ofi  cadmium.  Rutile,  the  mineral  with  the 
highest  specific  gravity,  forms  at  the  highest  temperature,  and  high  tempera- 
ture is  especially  characteristic  of  the  zone  of  anamorphism. 

Rutile  may  alter  into  ilmenite  (rhombohedral ;  sp.  gr.  4.75)  and  into 
titanite  (moiiocliuic;  sp.  gr.  3.48).  In  the  change  to  ilmenite  the  reactions 
may  be: 

(1)  TiO2+Fes04=FeTiO3+Fe,08+k,  or 

(2)  TiO2+FeCO3=FeTiO8+CO2+k. 

lu  (1)  the  decrease  in  volume  of  the  ilmenite  and  hematite  as  compared 
with  the  rutile  and  magnetite  is  1.88  per  cent.  In  (2)  the  decrease  in 
volume  is  34.77  per  cent,  provided  the  iron  carbonate  is  present  as  solid 
siderite  and  the  C02  escapes. 

In  the  change  to  titanite  the  most  probable  reaction  is: 

(3)  TiO2+CaCO3+Si02=CaTiSiO5+CO2+k. 

The  decrease  in  volume  is  28.17  per  cent,  provided  the  compounds  which 
unite  with  the  rutile  are  solids  and  the  liberated  C02  escapes. 

Those  who  have  described  the  changes  of  rutile  to  ilmenite  and  titanite 
have  not  indicated  whether  or  not  they  occur  as  deep-seated  alterations.  It 
may,  however,  be  anticipated  that  such  is  the  case,  for  they  are  changes 
which  involve  liberation  of  heat  and  condensation  of  volume,  and  therefore 
the  kind  which  normally  occur  in  the  zone  of  anamorphism. 


DIASPORE    GROUP. 
DUSPOKE  AND  1,1  HUM  IK. 


Diaspore: 

AIO(OH). 

Orthorhombic. 

Sp.  gr.  3.3-3.5. 
lAmonite: 

2Fe2O3 .3H2O. 

Amorphous. 

Sp.  gr.  3.6-4.00. 


"Dana,  J.  D.,  A  system  of  mineralogy;  Descriptive  mineralogy,  by  E.  S.  Dana;  Wiley  &  Sons, 
New  York,  6th  ed.,  1892,  p.  239. 


232  A  TREATISE  ON  METAMORPHISM. 

DIASPOKE. 

occurrence.— Diaspore  is  especially  found  in  the  serpentine-  or  chlorite- 
bearing  schists  and  gneisses  and  in  dolomites.  In  these  rocks  it  is  frequently 
associated  with  corundum.  Diaspore  has  been  recorded  as  a  constituent 
of  granite,  nepheline-syenite,  and  basaltic  rocks.  As  noted  on  other  pages, 
for  the  alterations  of  various  minerals  ill  the  zone  of  katamorphism,  espe- 
cially the  belt  of  weathering,  it  may  be  produced  as  one  of  the  alteration 
products  of  the  following  minerals:  Biotite,  corundum,  gibbsite,  haiiynite, 
muscovite,  nephelite,  noselite,  phlogopite,  scapolites,  sodalite. 

Alterations. — No  alterations  of  diaspore  are  recorded.  However,  it  is 
probable  that  where  diaspore  is  deposited  in  sedimentary  rocks  and  is  deeply 
buried,  so  as  to  undergo  alteration  in  the  zone  of  anamorphism,  it  passes 
into  corundum  (rhombohedral,  sp.  gr.  4.025);  or,  like  corundum,  unites 
with  other  bases  to  produce  such  minerals  as  spinel  (isometric,  sp.  gr. 
sillimanite  (orthorhombic,  sp.  gr.  3.235),  and  cyanite  (triclinic,  sp.  gr.  3.615); 
muscovite  (monoclinic,  sp.  gr.  2.88);  margarite  (monoclinic,  sp.  gr.  3.035); 
and  zoisite  (orthorhombic,  sp.  gr.  331).  So  far  as  the  hydrous  minerals 
muscovite,  margarite,  and  zoisite  are  formed,  the  water  may  have  been 
derived  from  the  diaspore,  which  contains  more  water,  and  thus  their  forma- 
tion be  really  a  process  of  dehydration  and  silication.  For  these  supposed 
reactions  the  equations  may  be: 

(1)  2AlO(OH)=Al20,+H20-k. 

(2)  2A1O(OH)  +MgC03=MgAl2O4+CO2+H2O-k. 

(3)  2AlO(OH)+SiO2=AL!SiO5+H20-k. 

(4)  6AlO(OH)+6SiO2+K2CO3=2H2KAl3Sis012+CO2+H2O-k. 

(5)  4A1O(OH)  +2SiO2+CaCO3==H2CaAl4Si2O12+CO2+H2O-k. 

( 6)  6A1O  (OH )  +6SiO2+4CaCO3= H2Ca)Al6Si6O26+4C02+2H.iO-k. 

Supposing  that  all  the  compounds  in  the  first  member  of  the  equation  are 
solids,  and  that  the  liberated  C02  and  H20  escape,  the  decrease  in  volume 
for  corundum  (1)  would  be  28.18  per  cent;  for  spinel  (2),  40.39  per  cent; 
for  sillimanite  (3),  13.52  per  cent;  for  cyanite  (3),  22.61  per  cent;  for 
muscovite  (4),  54.21  per  cent;  for  margarite  (5),  14.08  per  cent;  for  zoisite 
(6),  2i).44  per  cent.  All  take  place  with  absorption  of  heat.  Therefore 
all  of  these  reactions  are  characteristic  of  the  zone  of  anamorphism. 


occurrence. — Limonite  as  a  mineral  is  produced  either  as  an  original 
chemical    precipitate  or  by  the  alteration  of  other  minerals.     It  is  not, 


OCCURRENCE  AND  ALTERATIONS  OF  LIMONITE.  233 

i   as  known,  an  original  pyrogenic  constituent  of  the  igneous  rocks. 
The  uost    important  source  of  bodies  of  limonite  is  precipitation  from 
ir<  Hi-  Baring-  solutions,  especially  iron  carbonate.     For  iron  carbonate  the 
in  is — 

4FeC03+2O+3H20=2Fe203.3H2O+4CO2+k. 

e  >nd  important  source  for  limouite  is  the  oxidation  and  hydration  of 

ron  carbonate  in  rocks,  especially  siderite,  ankerite,  parankerite,  and 
in  >n  earing  limestone  or  dolomite.  The  source  next  in  importance  is  the 
oxid  ion  and  hydration  of  pyrite,  marcasite,  and  other  sulphides.  A  fourth 

ant  source  of  limonite  is  the  oxidation  arid  hydration  of  the  ferrous 
iron  if  silicates.  A  fifth  source  is  the  hydration  of  hematite.  A  sixth 
In  it  [important  source  is  the  oxidation  and  hydration  of  magnetite.  All  the 

>ns  involve  oxidation  or  hydration,  or  both,  and  therefore  take  place 
Avitl  the  liberation  of  heat.  In  the  production  of  limonite  from  iron 

late  there  is  an  important  contraction  of  volume;  in  the  other  cases 
the  )lume  of  the  limonite  is  greater  than  that  of  the  compounds  from 
whi(  it  is  derived.  All  the  above  reactions  producing  limonite  occur  in 
the  >ne  of  katamorphism,  and  the  controlling  factor  is  the  first  part  of 

Hoff's  law,  that  of  chemical  reactions  with  the  liberation  of  heat. 
Limnite  does  not  develop  in  the  zone  of  anamorphism. 

n  summary,  limonite  is  derived  from  the  following  minerals:  Actino- 
lite,  ukerite,  anthophyllite,  arfvedsonite,  biotite,  bronzite,  epidote,  garnet, 
gri-i  ilite,  griinerite,  hematite,  hornblende,  hypersthene,  ilmenite,  mag- 
ncti  ,  marcasite,  olivine,  parankerite,  pyrite,  pyrrhotite,  serpentine,  and 
side  r,e. 

^iterations. — The  important  alterations  of  limonite  are  into  hematite 
(rlu  ibohedral;  sp.  gr.  5.20-5.25)  and  siderite  (rhombohedral ;  sp.  gr. 
3.833.88).  Hematite  produced  from  limonite  may  be  earthy  or  crystalline. 
Tlu  eaction  is — 

2Fe2O3.3H2O=2FeA+3H2O-k. 

Tlu  lecrease  of  volume  is  37.78  per  cent.  The  change  is  therefore  one 
of  (  hydration,  reduction  of  volume,  and  crystallization.  The  transforma- 
tion akes  place  on  a  great  scale  in  the  zone  of  anamorphism,  that  in  which 
pres-ure  controls  whether  heat  is  absorbed  or  liberated. 

The  second  important  change  of  limonite  is  into  iron  carbonate. 
Whre  this  change  occurs  organic  compounds  are  commonly  present  and 


234  A  TREATISE  ON  METAMORPHiSM. 

decomposing  to  serve  as  reducing  agents  and  to  furnish  abundant  CO2  to 
unite  with  the  iron.  The  reduction  may  be  by  the  passage  of  CO  into 
C02,  of  C  into  C02,  or  of  C  into  CO,  as  follows: 

(1)  2Fe2O,.3H,O+2CO+2CO2=4FeC03+3H2O+k. 

(2)  2Fe2O8.3H20+C+3CO2=4FeCOs+3H.1O+k. 

(3)  2FeA-3H2O+2C+4C02=4FeCO3+3H2O+2CO+k. 

So  far  as  the  iron  is  concerned,  its  reduction  and  dehydration  absorb  heat, 
but  the  oxidation  of  the  C  or  CO  and  the  union  of  the  CO2  and  FeO  both 
liberate  heat,  the  amount  of  which  is  greater  than  that  absorbed,  so  that  in 
each  of  these  reactions  heat  is  liberated.  In  all  of  the  reactions  the  volume 
is  increased  22.27  per  cent. 

The  reduction  of  the  iron  of  limonite  so  as  to  produce  protoxide  for 
the  formation  of  iron  carbonate  may  of  course  be  accomplished  by  carbu- 
reted hydrogen,  especially  methane  (CH4),  rather  than  by  the  compounds 
suggested;  but  the  carbureted  hydrogen  compounds  are  so  numerous  and 
the  resultant  compounds  so  uncertain  that  no  attempt  will  be  made  to 
formulate  equations  for  possible  changes  with  these  substances  as  reducing 
agents. 

The  change  of  limonite  to  siderite  is  one  which  occurs  extensively  in 
rocks  bearing  organic  compounds  in  the  zone  of  katamorphism.  The 
formation  of  the  abundant  siderites  which  are  used  as  iron  ores  of  Carbon- 
iferous and  later  age  are  believed  for  the  most  part  to  be  thus  derived  from 
limonite  in  the  upper  zone.  The  reactions  correspond  perfectly  to  this 
position,  being  those  which  occur  with  liberation  of  heat  and  very  consid- 
erable expansion  of  volume.  The  siderite  thus  formed  may  later  be 
decomposed  into  various  other  compounds,  or  even  reproduce  limonite,  but 
the  consideration  of  such  changes  belongs  under  "Siderite." 


Brucite: 

Mg(OH)2. 

Rhombohedral. 

Sp.  gr.  2.38-2.40. 
Gibbgite  (hydrargillite): 

A1(OH), 

Monoclinie. 

Sp.  gr.  2.28-2.42. 


BEUCITE    GROUP. 
BBUCITE  AJiD  (JIBBS1TE. 


BRUCITE  AND  GIBBSITE.  235 

BKUCITE. 

occurrence. — Brucite  is  one  of  the  minerals  which  is  produced  in  the 
upper  physical-chemical  zone,  especially  in  the  belt  of  weathering.  Brucite 
is  produced  by  the  alterations  of  minerals  rich  in  magnesia,  being  recorded 
as  secondary  to  chondrodite,  clinohumite,  humite,  and  serpentine.  It  is 
especially  prevalent  in  serpentinous  rocks  and  veins.  Doubtless  in  many 
instances  it  forms  simultaneously  with  the  serpentine  and  perhaps  other 
minerals,  rather  than  secondary  to  them. 

Alterations. — The  one  alteration  of  brucite  noted  is  that  of  carbonation, 
into  hydromagnesite  (monoclinic;  sp.  gr.  2.145-2.180).  The  reaction 
representing  the  change  is— 

4Mg(OH)2+3CO2  =  Mg2(CO3),.2Mg(OH).3H.,O+k. 

The  increase  in  volume    is    73.08    per    cent.  The   alteration  is  therefore 

one  of  simple  carbonation,  and  takes  place  in  the  zone  of  katamorphism, 

especially   in   the    belt    of    weathering,    with  expansion    of    volume    and 
liberation  of  heat. 

GIBBSITE. 

occurrence. — Gibbsite  occurs  as  an  accessory  constituent  in  many  of 
the  schists  and  gneisses,  especially  those  which  have  been  subjected  to  the 
forces  of  the  upper  physical-chemical  zone,  and  particularly  in  the  belt  of 
weathering.  As  noted  on  subsequent  pages,  it  may  be  a  result  of  the 
alteration  of  many  minerals,  the  more  important  of  which  are  as  follows: 
Anorthoclase,  andalusite,  biotite,  cancrinite,  corundum,  cyanite,  epidote, 
haiiyiiite,  microcline,  muscovite,  nephelite,  noselite,  orthoclase,  phlogopite, 
plagioclases,  pyrope,  the  scapolites,  sillimanite,  sodalite,  topaz,  tourmaline, 
and  zoisite.  By  reference  to  the  discussion  of  these  minerals  and  the 
minerals  which  simultaneously  form,  the  conditions  of  its  formation  may 
be  ascertained. 

Alterations. — No  alterations  of  gibbsite  are  recorded  in  the  standard  text- 
books, but  where  sedimentary  rocks  containing  gibbsite  are  so  deeply 
buried  as  to  pass  into  the  zone  of  anamorphism  it  may  become  partly 
dehydrated,  producing  diaspore  (orthorhombic ;  sp.  gr.  3.40),  or  wholly 
dehydrated,  producing  corundum  (rhombohedral ;  sp.  gr.  4.025);  or  the 
aluminum  may  unite  with  other  compounds,  producing  the  same  minerals 
that  are  produced  by  corundum  or  diaspore.  It  is  believed  that  these 


236  A  TREATISE  ON  METAMORPHISM. 

alterations  from  diaspore  and  gibbsite  have  taken  place  on  an  extensive 
scale,  even  if  they  have  not  been  recorded.  There  is  no  doubt  about  the 
formation  of  gibbsite  abundantly  in  the  zone  of  katamorphism,  especially 
in  the  belt  of  weathering.  To  my  mind  there  is  as  little  doubt  that  the 
widespread  corundum  of  the  schists,  gneisses,  and  marbles  is  derived  in 
large  measure  from  gibbsite.  I  am  confident  that  the  hydrous  aluminum 
oxides  furnish  the  bases  for  much  of  the  spinel  (isometric;  sp.  gr.  3.80), 
sillimanite  (orthorhombic;  sp.  gr.  3.235),  and  cyanite  (triclmic;  sp.  gr- 
3.615)  which  occur  in  these  rocks.  And  it  is  little  short  of  a  certainty  that 
gibbsite  furnishes  alumina  for  the  silicates,  muscovite  (monoclinic;  sp.  gr. 
2.88),  margarite  (monoclinic;  sp.  gi\  3.035),  and  zoisite  (orthorhombic; 
sp.  gr.  3.31).  As  with  diaspore,  the  reactions  producing  all  the  above- 
mentioned  silicates  are  those  of  dehydration  and  silicifiation.  The  following 
equations  may  be  written  for  the  above  supposed  reactions: 

(1)  Al(OH)==AlO(OH)+H20-k. 

(2)  2A1(OH)3=A12OS+3H2O-  k. 

(3)  2Al(OH)3+MgC03=MgAl2O4+CO2+3H,O-k. 

(4)  2A1  (OH),+Si02=Al.,SiO6+3H2O-k. 

(5)  6Al(OH)8-fGSiO,+K2C03=2II2KAl3SisO12T-CO2+7H2O-k. 

(6)  4Al(OH)3+2SiO2+CaCO3=H.,CaAl4Si2O12+CO2+5H.,0-k. 

(7)  6Al(OH)s+6Si02+4CaC03=H2Ca(Al6Si6026+4C02+8H20-k. 

Regarding  all  the  minerals  as  solid,  the  decrease  of  volume  for  diaspore  (1) 
is  46.82  per  cent;  for  corundum  (2),  61.81  per  cent;  for  spinel  (3),  60.12 
per  cent;  for  sillimanite  (4),  43.68  per  cent;  for  cyanite  (4),  49.61  per  cent; 
for  muscovite  (5),  64.99  per  cent;  for  margarite  (6),  38.92  per  cent;  for 
zoisite  (7),  43.06  per  cent.  The  decreases  of  volume  are  greater  for  the 
corresponding  minerals  than  for  diaspore  because  of  the  greater  amount  of 
water  in  the  gibbsite.  In  all  the  reactions  heat  is  absorbed.  The  reactions 
are  therefore  typical  of  the  zone  of  anamorphism. 

THE  CARBONATES. 

The  important  carbonates  which  occur  as  rock-making  constituents 
are  the  calcite  group,  including  calcite,  dolomite,  ankerite  and  parankerite, 
magnesite,  and  siderite,  and  the  aragonite  group,  of  which  aragonite  is  the 
only  important  rock-making  member. 


OCCURRENCE  OF  CALCITE.  237 

CALCITE   GROUP. 
CALCITE,  DOLOMITE,  ANKERITE,  PABAXKERITE,  MAGNESITE,  AND  M  Ml  I;  I  I  I  . 

Calcite: 

CaCO3. 

Rhombohedral.  • 

Sp.  gr.  2.713-2.714. 
Dolomite: 

CaMgC2O6. 

Rhombohedral. 

Sp.  gr.  2.8-2.9. 
AnkerUe: 

CaFeC3O6.CaMgC2O6;(CaMgC2O6:CaFeC,O6::i:l  to  2:1). 

Rhombohedral. 

Sp.  gr.  2.95-3.1. 
Parankerite: 

CaFeC2O6.2CaMgC.,O6;(CaMgC2O6:CaFeC2O6::2:l  to  10:1^ 

Rhombohedral. 

Sp.  gr.  2.95-3.1. 
Magnesile: 

MgC03. 

Rhombohedral. 

Sp.  gr.  3.00-3.12. 
Siderite: 

FeCO3. 

Rhombohedral. 

Sp.  gr.  3.83-3.88. 

CALCITK. 

occurrence. — The  chief  sources  of  calcite  are  (1)  organic  precipitates,  (2) 
chemical  precipitates,  (3)  by  alteration  of  aragouite,  and  (4)  by  carbonation 
of  silicates. 

The  chief  direct  source  of  calcite  is  organic.  Corals  and  innumerable 
other  kinds  of  shell  animals,  especially  in  the  sea,  abstract  calcium  carbonate 
from  the  water  and  build  it  into  their  external  or  internal  structures.  Calcite 
as  a  chemical  precipitate  may  be  deposited  from  the  waters  of  the  sea, 
especially  in  inclosed  lagoons;  by  the  waters  of  inland  lakes,  especially 
those  having  no  outlet;  by  springs  and  streams,  especially  hot  springs  and 
desert  streams;  and  by  underground  waters  in  the  openings  of  rocks,  such 
as  the  interstices  between  grains,  the  cavities  of  porous  igneous  rocks, 
especially  amygdules,  and  in  cave,  fault,  joint,  and  fissility  openings.  The 
deposited  calcite  may  replace  a  considerable  number  of  other  minerals.  As 
a  deposit  in  the  openings  of  rocks  calcite  is  second  in  abundance  only  to 
quartz.  Calcite  is  an  alteration  product  of  a  large  number  of  minerals,  of 
which  the  following  are  the  more  common:  Actinolite,  ankerite,  antho- 


238  A  TREATISE  ON  METAMORPHISM. 

phyllite,  aragonite,  augite,  diopside,  dolomite,  epidote,  fluorite,  garnet, 
grossularite,  gypsum,  haiiynite,  hornblende,  noselite,  parankerite,  pyrope, 
sahlite,  scapolites,  tremolite,  and  zoisite. 

While  the  abundant  direct  sources  of  calcite  are  (1),  (2),  and  (3) 
above,  the  indirect  and  ultimate  source  which  has  probably  furnished  the 
great  quantity  of  calcium  carbonate  is  the  carboiiation  of  the  silicates.  (See 
pp.  473-480.)  This  process  occurs  on  a  great  scale  in  the  zone  of  kata- 
morphism,  especially  in  the  belt  of  weathering.  It  is  a  reaction  which 
takes  place  with  liberation  of  heat  and  increase  of  volume  in  case  the 
replaced  silica  separates  as  quartz  in  situ.  Many  of  the  individual  carbo- 
nation  reactions  of  the  silicates,  as,  for  instance,  wollastouite,  diopside,  etc., 
are  given  under  that  class  of  minerals. 

Alterations. — The  first  of  the  alterations  of  calcite  is  recrystallization.  Cal- 
cite is  the  most  mobile  of  the  abundant  rock-making  minerals.  It  responds 
readily  to  changes  of  physical  conditions,  and  is  very  susceptible  to  weak 
chemical  agents.  A  slight  stress  may  produce  in  it  twinning  structure.  A 
state  of  unequal  strain  favors  its  solubility.  Where  the  pressure  increases, 
solution  increases;  where  pressure  is  lessened,  deposition  takes  place. 
Increase  of  temperature  greatly  increases  its  solubility,  and  vice  versa. 
The  increase  of  carbon  dioxide  in  water  greatly  increases  its  solubility, 
and  vice  versa.  Thus  it  happens  that  in  rocks  where  the  calcite  is  almost 
constantly  subjected  to  changing  pressure,  temperature,  and  varying 
amounts  of  carbon  dioxide  it  is  constantly  being  taken  into  solution 
and,  after  a  greater  or  less  journey,  being  deposited  from  solution  or 
carried  to  the  sea  to  be  ultimately  precipitated  by  organic  agents.  The 
recrystallization  of  great  masses  of  calcite,  the  solution  of  calcite  in  the 
belt  of  weathering  and  its  partial  deposition  in  the  belt  of  cementation,  the 
formation  of  caves,  cave  deposits,  etc.,  are  considered  later. 

The  second  important  change  of  calcite  is  partial  replacement  of  cal- 
cium by  magnesium,  often  producing  dolomite  (rhombohedral ;  sp.  gr. 
2.8-2.9).  The  generalized  reaction  is: 

(1)  2CaCOs-fMg=CaMg(CO,)2+Ca+k. 

Supposing  the  calcium  to  be  present  as  a  carbonate,  and  supposing  the 
added  magnesium  to  be  a  chloride — and  this  is  believed  to  be  a  very 
common  case — the  reaction  would  be: 

(2)  2CaCOs+MgCl2=CaMgC.A+CaCl.,!+k. 


ALTERATIONS  OF  CALCITE.  239 

Or  supposing  that  the  magnesium  salt  is  a  carbonate,  and  that  this  is  depos- 
ited and  an  equivalent  amount  of  calcium  carbonate  is  taken  into  solution, 
the  reaction  would  be: 

(3)  2CaCO8+MgCO3=CaMgO2O6+CaCOs+k. 

Either  of  these  changes  is  accompanied  by  the  decrease  in  volume  of  12.30 
per  cent  if  the  original  calcite  be  compared  with  the  produced  dolomite. 
There  might  be  no  diminution  in  volume,  or  even  an  increase  in  volume, 
in  case  less  than  the  molecular  weight  of  calcium  salt  equivalent  to  the 
introduced  magnesium  was  dissolved.  For  instance,  in  an  extreme  case 
the  reaction  might  be: 

(4)  CaCOs+MgCOs=CaMg(CO,)2+k, 

the  MgC03  being  added  through  solutions,  and  no  calcium  carbonate 
dissolved.  In  this  case  the  expansion  in  volume  over  the  original  calcite 
would  be  very  great — 75.41  per  cent.  However,  the  normal  case  in 
dolomitization,  as  noted  below,  appears  to  be  the  molecular  replacement 
represented  by  the  specific  equations  (2)  and  (3).  The  compounds 
concerned  in  these  reactions  are  so  important  that  the  heat  relations  have 
been  determined  as  above  given;  so  it  can  be  asserted  positively,  from 
chemical  studies,  that  heat  is  liberated  by  them. 

The  calcium  of  calcium  carbonate  may  be  replaced  by  other  metals 
besides  magnesium,  or  calcite  may  be  replaced  by  an  oxide.  The  most 
important  of  the  elements  which  enter  into  such  combinations,  and  the 
only  one  which  need  be  mentioned,  is  iron.  At  many  localities,  partly 
or  wholly  occupying  the  place  once  held  by  calcite,  iron  carbonate  is 
found.  For  any  definite  proportion  of  iron  replacing  the  calcium,  equations 
may  be  written  paralleling  those  for  the  replacement  of  calcium  by 
magnesium. 

The  third  important  alteration  of  calcite  is  to  wollastonite  (monoclinic ; 
sp.  gr.  2.8-2.9).  This  alteration  is,  indeed,  the  chief  source  of  wollastonite. 
The  equation  is: 

(5)  CaCO3+SiO2=CaSiO3+CO2-k. 

In  the  change  the  volume  is  decreased  31.48  per  cent,  provided  the  silica 
used  is  a  solid  and  the  carbon  dioxide  escapes.  In  case  the  silicic  acid  be 
brought  in  solution  from  an  outside  source,  the  volume  of  the  solid  is 
increased  10.81  per  cent.  Between  these  extremes  there  are  theoretically 


240  A  TREATISE  ON  METAMORPHISM. 

all  gradations,  but,  as  noted  below,  an  approach  to  the  former  extreme 
probably  is  the  common  case. 

Recrystallizatiou  of  calcite  and  dolomitizatioii  take  place  on  the  most 
extensive  scale  at  all  depths  and  under  both  mass-static  and  mass-dynamic 
conditions;  they  are  therefore  alterations  which  are  common  to  both 
physical-chemical  zones.  By  dolomitization  it  is  believed  that  great  masses 
of  calcite  have  been  transformed  to  dolomite.  The  evidence  of  this  trans- 
formation and  the  detailed  facts  in  connection  with  the  change  are  given 
under  dolomite.  (See  pp.  798-808.)  The  fact  that  dolomite  forms  in  both 
zones  would  be  sufficient  evidence  that  the  reactions  producing  this  com- 
pound liberate  heat,  even  if  this  had  not  been  experimentally  determined 
to  be  the  fact,  It  has  been  pointed  out,  before  (pp.  181-182)  that  the 
formation  of  dolomite  is  a  typical  illustration  of  an  alteration  in  which  both 
the  volume  and  the  chemical  changes  liberate  heat,  and  which  therefore 
may  occur  in  all  zones  and  belts  of  the  lithosphere. 

The  change  from  calcite  to  wollastonite  occurs  chiefly  or  wholly  in 
the  very  deep-seated  rocks,  especially  in  the  zone  of  anamorphism.  In  this 
zone,  as  noted  (pp.  764-766),  it  can  not  be  assumed  that  material  is  added 
in  considerable  quantity  from  an  outside  source  by  circulating  water;  hence 
in  this  zone  silica  for  the  change  is  believed  to  have  been  a  solid.  The 
reaction  is  therefore  one  taking  place  with  the  absorption  of  heat  and 
condensation  of  volume.  The  silication  of  calcite  to  wollastonite  in  the 
zone  of  anamorphism  may  be  taken  as  a  typical  example  of  the  heat  and 
volume  change  of  silication  of  carbonates  in  that  zone. 

DOLOMITE. 

occurrence. — The  chief  source  of  dolomite  is  believed  to  be  the  dolomiti- 
zation of  calcite  (see  pp.  238-239),  but  dolomite  is  also  a  direct  chemical 
precipitate.  Dolomite  also  forms  in  subordinate  amount  by  the  alteration 
of  ankerite.  The  ultimate  source  of  the  magnesium  carbonate  for  the 
dolomitization  of  the  calcite  is  the  magnesium  liberated  by  the  carboua- 
tion  of  the  silicates  in  the  zone  of  katamorphism,  especially  in  the  belt 
of  weathering.  The  reactions  for  the  decomposition  of  some  of  the 
simple  silicates,  such  as  diopside  and  tremolite,  are  given  under  those 
minerals.  The  magnesium  for  the  dolomitization  need  not  be  directly 
derived  from  a  silicate,  but  may  be  from  the  solutions  of  the  sea  or  from 


ALTERATIONS  OF  DOLOMITE.  241 

a  previously  formed  magnesium  limestone  or  dolomite  which  is  in  the 
belt  of  weathering.  Dolomite  produced  by  the  carbonation  of  the  silicates 
or  by  solution  of  dolomitio  formations  is  an  important  chemical  precipitate 
in  caves  and  small  crevices  in  the  rocks,  the  same  as  calcite. 

In  summary,  dolomite  is  chiefly  derived  as  a  secondary  mineral  from 
ankerite,  calcite,  and  parankerite. 

Alterations. — An  important  alteration  of  dolomite  is  to  diopside  (mono- 
clinic;  sp.  gr.  3.2-3.38).  This  alteration  is  a  typical  example  of  silication. 
(See  p.  205.)  The  most  probable  reaction  is: 

( 1 )  MgCaC2O6+2Si02= MgCaSi2O6+2CO2  -  k. 

The  decrease  in  volume  is  40.11  per  cent,  provided  all  of  the  silica  entering 
into  the  combination  was  a  solid.  In  case  all  of  the  silica  were  introduced 
through  water  solutions  there  would  be  an  increase  in  volume  of  2.03  per 
cent.  More  important  alterations  of  dolomite  are  into  trernolite  (mono- 
clinic;  sp.  gr.  2.9-3.1)  and  calcite  (rhombohedral ;  sp.  gr.  2.713-2.714), 
or  into  tremolite  and  wollastonite  (monoclinic;  sp.  gr.  2.8-2.9).  In  the  first 
case  the  reaction  is: 

(2)  3CaMgC2O6+4SiO2=Mg3CaSi4O12+2CaCO3+4CO.,-k. 

The  decrease  in  volume,  provided  the  silica  is  present  as  a  solid,  the 
calcite  remains  as  a  solid,  and  the  carbon  dioxide  escapes,  is  25.20  per  cent. 
However,  the  excess  of  calcium  carbonate  may  simultaneously  change  to 
wollastonite.  In  this  case  the  reaction  would  be: 

(3)  3CaMgC2O6+6SiO2=Mg3CaSi4O12+2CaSiO3+6CO2--k. 

The  decrease  in  volume  as  compared  with  the  dolomite  and  quartz  of  the 
tremolite  and  wollastonite  is  33.09  per  cent.  In  both  of  the  changes,  if  a 
portion  of  the  silica  be  supposed  to  be  introduced  from  an  outside  source 
the  decrease  in  volume  would  be  lessened,  and  if  all  of  it  were  thus  sup- 
posed to  be  introduced  there  would  be  an  increase  in  volume  from  the 
solid  dolomite  of  9.89  per  cent  in  the  case  of  tremolite  and  calcite,  and  14 
per  cent  in  the  case  of  tremolite  and  wollastonite. 

The  space  once  occupied  by  dolomite,  like  that  occupied  by  calcite, 
may  be  taken  by  other  carbonates  or  by  various  oxides.     The  most  impor- 
tant of  these  are  carbonate  of  iron  and  oxide  of  iron.     The  carbonate  may 
be  a  replacement,  or  possibly  a  substitution,  of  the  iron  of  some  other  iron 
MON  XLVII — 04 16 


242  A  TREATISE  ON  METAMORPHISM. 

salt  for  that  of  the  calcium  and  magnesium.     The  oxide  of  iron  is  an  illus- 
tration of  a  pure  replacement,  not  of  an  alteration. 

The  formation  of  diopside,  tremolite,  and  wollastonite  is  known  to 
occur  in  deep-seated  rocks,  and  especially  in  connection  with  mass- 
mechanical  action  where  the  rocks  are  deformed  by  flowage.  As 
repeatedly  noted,  in  the  zone  of  anamorphism  the  circulation  of  water  is 
reduced  to  a  minimum;  and  it  can  not  be  supposed  that  important  addi- 
tions are  made  from  the  outside,  and  therefore  the  silica  must  be  supposed 
to  have  been  previously  present  in  the  rocks.  Indeed,  we  know  that  silica 
usually  accompanies  deposits  of  calcite  and  dolomite ;  hence  I  conclude  that 
the  reactions  take  place  with  substantially  the  decrease  in  volume  above 
assigned  to  the  changes.  In  the  reactions  heat  is  absorbed.  The  changes 
are  therefore  again  typical  illustrations  of  silication  in  the  lower  physical- 
chemical  zone. 

ANKERITE   AND   PARANKERITE. 

occurrence. — All  the  compounds  from  normal  ankerite  and  parankerite  to 
the  extremes  of  composition  given  above  (p.  237)  are  included  under  the 
general  term  ferro-dolomite,  which  I  have  elsewhere  used  as  covering  all 
the  ferriferous  compounds  standing  between  dolomite  on  the  one  side  and 
siderite  on  the  other.  (See  p.  823.) 

The  sources  of  ankerite  and  parankerite  are  the  same  as  siderite, 
with  the  difference  that  at  the  time  of  the  formation  of  the  iron  carbonate, 
calcium  and  magnesium  carbonate  are  present,  or  formed,  and  unite  with  it. 

Alterations. — The  more  common  alterations  of  ankerite  and  parankerite 
are  to  limonite  (amorphous;  sp.  gr.  3.80),  hematite  (rhombohedral ;  sp. 
gr.  5.225),  and  magnetite  (isometric;  sp.  gr.  5.174),  the  calcium  and 
magnesium  carbonates  either  separating  or  simultaneously  undergoing  the 
alterations  given  under  "Calcite"  and  "Dolomite."  Equations  may  easily 
be  written  for  any  definite  compound  by  which  the  iron  carbonate  passes 
into  the  minerals  mentioned  in  the  same  way  that  siderite  does  and  the 
calcium-magnesium  carbonates  separate.  The  volume  changes  are  in  the 
same  direction,  and  the  physical  conditions  under  which  aukerite  and  par- 
ankerite alter  to  limonite,  hematite,  and  magnetite  are  the  same  as  those 
for  the  alteration  of  siderite  to  the  like  compounds.  Therefore  the  equa- 
tions and  summary  of  physical  conditions  will  not  be  here  repeated. 

Other  important  alterations  of  the  ferro-dolomites  are  to  sahlite  (mono- 


ALTERATIONS  OF  ANKERITE  AND  PARANKERITE.  243 

clinic;  sp.  gr.  3.25—3.4)  and  to  actinolite  (rnonoclinic;  sp.  gr.  3.00-3.20). 
Supposing  that  the  magnesium  and  iron  are  present  in  equal  quantity  in 
the  sahlite,  the  reaction  in  the  case  of  normal  ankerite  is : 

( 1 )  CaFeCA.CaMgCA+4SiO2=AIgFeCa2Si4O12+4CO2-k. 

Supposing  the  silica  to  be  present  as  a  solid,  the  decrease  in  volume  is 
37.27  per  cent.  In  the  formation  of  actinolite  from  normal  aukerite,  on 
the  supposition  that  the  iron  and  magnesium  are  present  in  equal  quantity 
in  the  actinolite,  the  reaction  is: 

(2)  SCaFeCA.CaMgCA+SSiO^MgsFesCa^SiAi+^CaCOs+SCOj-k. 

The  decrease  in  volume,  supposing  the  silica  to  be  present  as  a  solid  and 
the  CaC03  as  a  solid,  is  '22.Q2  per  cent.  Of  course,  if  the  ferro-dolomite 
were  one  in  which  the  calcium  carbonate  is  not  so  plentiful,  being  replaced 
in  equal  molecular  parts  by  magnesium  and  iron,  it  would  not  be  necessary 
for  any  calcium  carbonate  to  form  as  a  result  of  the  reaction.  For  instance, 
if  the  ferro-dolomite  were  CaFeaC^D^.CaMgsC^a  the  reaction  would  be  as 
follows : 

( 3)  CaFesC  A,.  CaMgAO,., + 8SiO2 = Mg3FesCa,Si8O24+  8CO2  -  k . 

Using  the  specific  gravity  of  normal  ankerite,  the  decrease  of  volume  of 
the  actinolite  as  compared  with  the  ankerite  and  quartz  is  32.72. 

Sahlite  and  actinolite  are  both  known  to  form  abundantly  in  the  zone 
of  anamorphism.  Sahlite  is  found  in  the  marbles  of  eastern  United  States. 
Actinolite  is  very  abundant  iir  the  iron-bearing  formations  of  the  Lake 
Superior  region.  The  development  of  these  silicates  may  be  taken  as 
typical  illustrations  of  the  reaction  of  silication  in  the  lower  physical- 
chemical  zone,  with  condensation  of  volume  and  absorption  of  heat. 


MAONESITE. 


occurrence. — Magiiesite  may  be  a  product  of  the  alteration  of  any  of  the 
heavily  magnesian  rocks.  It  is  especially  prevalent  in  the  olivinitic 
rocks  and  the  chloritic,  serpentinous,  and  talcose  schists  and  gneisses, 
being  a  product  which  is  produced  .by  the  alteration  of  original  minerals 
simultaneously  with  the  formation  of  chlorite,  serpentine,  and  talc.  It 
is  also  found  in  dolomite.  The  more  important  minerals  from  which  it  is 
recorded  as  forming  are  common  garnet,  olivine,  pyrope,  and  serpentine. 

Alterations. — No  alterations  are  recorded  for  magnesite.  There  is,  how- 
ever, no  doubt  that  this  compound  does  break  up  in  the  zone  of  aiiamor- 


244  A  TREATISE  ON  METAMORPHISM. 

ism,  the  carbon  dioxide  being  liberated  and  the  magnesia  being  furnished 
for  the  formation  of  various  dense  magnesian  minerals,  such  as  enstatite, 
tremolite,  olivine,  pyrope,  etc.  These  changes  would  involve  a  diminution 
of  volume  and  an  absorption  of  heat. 


occurrence. — The  chief  source  of  siderite  is  believed  to  be  the  reduction, 
dehydration,  and  carbonation  of  limonite.  (See  pp.  233-234.)  This  change 
is  one  occurring  with  the  liberation  of  heat  if  the  reaction  upon  the  organic 
compound  be  taken  into  account,  and  increase  of  volume.  A  subordinate 
amount  of  siderite  is  also  derived  from  magnetite.  This  change  takes 
place  with  liberation  of  heat  and  increase  of  volume.  Siderite  also  forms 
from  ankerite  and  parankerite,  arfvedsouite,  garnet,  hematite,  hornblende, 
hydrous  ferrous  silicate,  limonite,  magnetite,  and  olivine,  and  replaces 
calcite  and  dolomite. 

Alterations. — The  important  alterations  of  siderite  are  into  limonite 
(amorphous;  sp,  gr.  3.6-4.0),  hematite  (hexagonal-rhombohedral ;  sp.  gr. 
5.225),  magnetite  (isometric;  sp.  gr.  5.168-5.18),  and  griinerite  (monoclinic; 
sp.  gr.  3.713).  The  reactions  are  as  follows: 

(1)  4FeCOs+2O+3HJ0=2Fe,Os.3H20+4CO.!+k. 

The  decrease  in  volume  is  18.22  per  cent. 

(2)  2FeCO,+O=Fe2Os+2COark. 

The  decrease  in  volume  is  49.11  per  cent. 

(3)  3FeCOs+0=Fe,O4+3C02+k. 

Very  often  iron  sulphide,  as  pyrite  (isometric;  sp.  gr.  5.025)  or 
marcasite  (orthorhombic ;  sp.  gr.  4.875),  unites  with  the  siderite  to  form 
magnetite.  This  reaction  is  probably  of  great  consequence  in  forming  the 
heavy  beds  of  magnetite.  (See  p.  845.)  It  may  be  written: 

(4)  2FeCOs+FeS2+2HaO=Fe,04+2H2S42C02-k. 

The  decrease  in  volume  for  the  siderite  alone  to  the  magnetite,  equation 
(3),  is  50  32  per  cent;  for  siderite  and  pyrite,  46.67  per  cent;  for  siderite 
and  marcasite,  47.135  per  cent. 

(5)  FeCOs+SiO,+nH20=FeSi08+C02+nH20-k. 

The  decrease  in  volume,  regarding  the  silica  as  a  solid,  is  32  53  per  cent. 
The  alteration  to  limouite  occurs  in  the  zone  of  katamorphism,  especially 


OCCURRENCE  AND  ALTERATIONS  OF  ARAGONITE.  245 

in  the  belt  of  weathering.  The  alteration  to  hematite  occurs  as  a  somewhat 
deeper  seated  change,  usually  in  the  belt  of  cementation  of  the  zone  of 
katamorphism.  The  alteration  to  magnetite  is  especially  characteristic  of 
the  zone  of  anamorphism,  but  it  can  not  be  asserted  not  to  take  place  in 
the  belt  of  cementation.  The  alteration  to  griinerite  occurs  under  deep- 
seated  conditions,  and  is  in  its  heat  and  volume  relations  a  characteristic 
reaction  of  the  lower  zone.  Magnetite  and  griinerite  often  form  simul- 
taneously. (See  p.  284 )  The  series  of  changes  from  siderite  are  very 
interesting,  in  that  the  volume  changes  are  all  diminutions,  and  therefore, 
so  far  as  this  factor  is  concerned,  might  take  place  in  either  zone.  The  first 
three  reactions  (equations  1,  2,  and  3)  liberate  heat,  and  hence  these  reac- 
tions in  their  physical -chemical  relations  are  similar  to  those  of  dolomite, 
discussed  on  pages  182,  240,  and  may  take  place  in  both  zones.  But  the 
reaction  of  equation  (4)  probably  absorbs  heat,  and  that  of  (5)  certainly  does. 
Magnetite  having  the  origin  represented  by  equation  (4)  is  probably,  and 
griinerite  is  certainly,  confined  to  the  zone  of  anamorphism,  where  pressure 
is  a  controlling  factor. 

ARAGONITE   GROUP. 

The  only  important  rock-making  member  of  this  group  is  aragonite. 

AIUtiOMTE. 

Aragonite: 
CaCO3. 

Orthorhombic. 
Sp.  gr.  2.93-2.95. 

occurrence. — A  chief  source  of  aragonite  is  as  an  organic  precipitate. 
It  occurs  intimately  associated  with  calcite  in  numerous  marine  shells. 
While  abundant,  it  is  very  subordinate  to  calcite  as  an  organic  deposit.  A 
second  abundant  source  of  aragonite  is  as  a  chemical  precipitate,  frequently 
in  association  with  beds  of  iron  carbonate  and  gypsum.  It  also  occurs  as 
a  chemical  precipitate  from  ground-water  solutions,  in  openings  in  rocks, 
especially  at  places  where  the  temperature  of  the  solutions  is  from  30°  to 
100°  C.  or  more.  Aragonite  is  not  mentioned  as  an  alteration  product 
of  other  minerals. 

Alterations. — The  chief  change  of  aragonite  is  to  calcite  (rhombohedral ; 
sp.  gr.  2.713—2.714).  This  is  a  change  involving  recrystallization,  increase 
of  symmetry,  and  lowering  of  specific  gravity.  The  increase  in  volume  is 
8.35. per  cent.  The  heat  effect  of  the  change  has  not  been  found;  but  it 


246  A  TREATISE  ON  METAMOKPHISM. 

seems  probable  that  heat  is  liberated,  for  the  transformation  of  aragonite  to 
calcite  occurs  in  both  the  physical-chemical  zones,  and  I  know  of  no  excep- 
tion to  the  principle  that  such  reactions  take  place  under  the  first  part  of 
van't  Hoff's  law  (see  pp.  107,  181). 

The  change  from  aragonite  to  calcite  is  so  complete  in  rocks  of  mod- 
erate age  that  the  presence  of  aragonite  in  the  metamorphosed  rocks  is 
almost  unknown.  The  alteration  of  aragonite  to  calcite  in  both  zones  is  of 
considerable  interest,  as  it  presents  a  somewhat  exceptional  case.  As 
explained  on  pages  182-186,  the  common  rule  of  change  in  the  zone  of 
anamorphism  is  increase  in  specific  gravity  and  increase  of  symmetry, 
provided  the  volume  change  demanded  will  allow  this.  However,  the 
change  of  specific  gravity  in  this  case  is  a  decrease  rather  than  an  increase, 
and  hence  aragonite  conforms  only  to  the  second  of  these  rules — the  first, 
and  usually  the  controlling  rule,  for  the  zone  of  anamorphism  being 
violated.  These  facts  suggest  the  conclusion  that  in  this  instance  sym- 
metry is  a  more  important  factor  than  density — a  very  exceptional  thing. 
If  this  be  so,  the  conclusion  would  follow  that  the  symmetrical  arrange- 
ment of  the  molecules  in  calcite  are  those  which  best  resist  the  changing 
conditions  of  mass-static  and  mass-mechanical  action  in  the  lower  zone 
The  suggestion  occurs  to  one  that,  if  rocks  were  very  deeply  buried,  so  as 
to  be  extraordinarily  deep  in  the  lithosphere,  pressure  might  control  the 
form,  and  calcite  alter  to  aragonite.  This,  however,  is  a  speculation  which 
has  no  verification. 

THE  SILICATES. 

The  silicates  are  the  most  important  of  rock-making  constituents. 
They  include  natural  glass  and  many  mineral  groups.  The  groups  of 
rock-making  silicates  are  as  follows:  Feldspar,  leucite,  pyroxene,  amphi- 
bole,  nephelite,  sodalite,  garnet,  chrysolite,  scapolite,  zircon,  aluminum- 
silicate,  epidote,  humite,  zeolite,  mica,  clintonite,  chlorite,  serpentine-talc, 
and  kaolin.  Besides  the  members  of  the  above  groups  are  a  number  of 
important  rock-making  silicates  not  so  included. 


GLASS. 


Glass,  while  not  a  definite  silicate  or  ordinarily  included  among  the 
specific  minerals,  is  an  important  rock- making  constituent,  and  therefore  must 
be  treated  in  connection  with  the  silicates  in  a  treatise  on  metamorphism. 


DEVITRIFICATION  OF  GLASS.  247 

occurrence.— Natural  glass  is  an  abundant  constituent  of  the  effusive  rocks. 
It  is  especially  prevalent  in  the  more  acid  ones,  but  is  not  confined  to  them, 
being  not  infrequently  abundant  in  the  intermediate  rocks,  such  as  basalts. 
A  lava  or  tuff  may  be  almost  wholly  composed  ot  glass,  or  glass  may  con- 
stitute but  a  small  part  of  the  background.  There  are  thus  all  gradations 
between  completely  crystalline  rocks  and  glassy  rocks.  Of  the  more  recent 
effusive  rocks  glass  not  infrequently  composes  a  large  part  of  the  flows. 
An  instance  is  Obsidian  Cliff,  in  the  Yellowstone  National  Park.  But  in 
proportion  as  lavas  are  old,  glass  is  less  and  less  likely  to  be  found,  and  in 
the  more  ancient  lavas  is  ordinarily  absent.  The  explanation  of  this 
absence  is  devitrification  after  solidification. 

Evidence  that  devitrification  takes  place. That  Certain  1'Ocks   HOW    wholly  COmpOSed 

of  minerals  were  once  glasses  is  shown  by  the  preservation  in  perfection  of 
the  flow  structures  and  very  delicate  trichitic,  perlitic,  spherulitic,  and  other 
textures  characteristic  of  glass. 

scale  of  devitrification. — It  is  also  certain  that  the  process  of  devitrification 
has  taken  place  in  nature  on  a  great  scale.  As  evidence  of  this  may  be 
cited  the  well-known  American  instances  of  devitrified  glass  in  the  original 
Huronian  district,  described  by  Williams,"  the  aporhyolite  of  South  Moun- 
tain, Pennsylvania,  described  by  Williams  and  Bascom,*  the  metarhyolites 
of  the  Fox  River  Valley  of  Wisconsin,  described  by  Weidman,"  and  the 
devitrified  glasses  of  the  Crystal  Falls  district  of  Michigan,  described  by 
Clements.rf  In  the  papers  of  these  authors  many  other  instances  of 
devitrification  are  cited,  including  European  instances. 

Not  only  does  devitrification  of  natural  glass  take  place,  but  under 
proper  conditions  artifical  glass  devitrifies  in  a  similar  manner.  Well- 
known  cases  of  the  devitrification  of  artificial  glass  under  conditions  of 
weathering  are  those  of  the  buried  ancient  glasses  of  Nineveh  and  of  Rome. 

"Williams,  G.  H.,  Notes  on  the  microscopical  characters  of  rocks  from  the  Sudbury  mining 
district,  Canada:  Ann.  Kept.  Geol.  and  Nat.  Hist.  Survey  of  Canada,  vol.  5,  Pt.  F,  Appendix  1,  1890- 
1891,  pp.  74-82. 

*  Williams,  G.  H.,  The  volcanic  rocks  of  South  Mountain,  in  Pennsylvania  and  Maryland:  Am. 
Jour.  Sci.,  3d  ser.,  vol.  44,  1892,  pp.  486-490. 

.Bascom,  Miss  Florence,  The  ancient  volcanic  rocks  of  South  Mountain,  Pennsylvania:  Bull.  U.  S. 
Geol.  Survey  No.  136,  1896,  pp.  42-61. 

«  Weidman,  Samuel,  A  contribution  to  the  geology  of  the  pre-Cambrian  igneous  rocks  of  the  Fox 
River  Valley,  Wisconsin:  Bull.  Wisconsin  Geol.  and  Nat.  Hist.  Survey  No.  3,  1898,  pp.  4-31. 

^Clements,  J.  Morgan,  and  Smyth,  H.  L.,  The  Crystal  Falls  iron-bearing  district  of  Michigan: 
Mon.  U.  S.  Geol.  Surv.,  vol.  36,  1899,  pp.  87,  101-103,  138. 


248  A  TREATISE  ON  METAMORPHISM. 

In  glass  found  in  the  lake  at  Walton  Hall,  near  Wakefield,  Bingley0  found 
that  the  alkalies  had  been  wholly  removed  by  decay.  Another  case  of 
devitrification  largely  clue  to  original  state  of  strain  is  the  glass  of  certain 
old  buildings,  such  as  cathedrals.  A  well-known  instance  is  that  of  St. 
Andrew's  Chapter  House.6 

Rate  of  devitrification. — The  rate  of  devitrification  of  glass  depends,  among 
other  things,  upon  (1)  composition,  (2)  strain  or  lack  of  strain,  (3)  pressure, 
(4)  mass-mechanical  action,  (5)  temperature,  (6)  moisture. 

In  any  given  case  of  devitrification  several  and  sometimes  all  of  these 
factors  enter,  and  hence  it  is  impossible  to  discriminate  the  effect  of  each. 
Very  often  devitrification  has  been  described  as  hydro-metamorphism,  but 
by  this  no  more  can  be  meant  than  that  water  is  usually  an  important 
factor  in  the  process. 

(1)  The  rate  of  devitrification  of  glass  increases  with  its  basicity.     This 
follows  from  the  ready  solubility  of  basic  glasses.     It  has  also  been  deter- 
mined that  glasses  rich  in  soda  devitrify  faster  than  those  rich  in  potash. 
This  corresponds  with  the  fact  emphasized  in  another  place  (see  p.  516) 
that  minerals  rich  in  soda  are  more  readily  decomposed  than  those  rich 
in  potash. 

(2)  It  is  shown  in  another  place  that  a  state  of  strain  in  minerals 
promotes  alteration.     (See  pp.  95-98.)     The  same  is  true  of  glass.     It  is 
definitely  known  that  unannealed  glass,  which  therefore  cooled  irregularly 
and  is  in  a  state  of  strain,  independently  of  pressure  or  movement  may 
partly  devitrify  in  a  few  years.     For  instance,  drawn-glass  tubing,  such  as 
is  used  in  the  chemical  laboratory,  if  kept  for  a  few  years  may  devitrify  so 
as  to  become  useless.     Another  well-known  case  of  devitrification  probably 
due  to  strain  is  the  glass  of  certain  cathedral  windows.     As  large  masses 
of  glass  cool  under  natural  conditions,  they  must  often  be  almost  at  the 
extreme  of  the  unannealed  condition,  and   therefore  in   a  high  state  of 
strain.     So  far  as  glass  is  in  this  condition,  even  without  reference  to  any 
extraneous    pressure  or  movement,   there  is  a  marked    tendency  toward 
devitrification.     The  stage  of  the  process  due  to  this  cause  is  dependent 
upon  the  amount  of  strain  and  the  time. 

«  Bingley,  C.  W.,  On  the  peculiar  action  of  mud  and  water  on  glass,  as  more  especially  illustrated 
by  some  specimens  of  glass  found  in  the  lake  at  Walton  Hall,  near  Wakefield:  Rept.  Twenty-eighth 
Meeting  British  Assoc.  Adv.  Sci.,  London,  1859,  pp.  45-46. 

^Brewster,  Sir  David,  On  the  decomposition  of  glass:  Rept.  Tenth  Meeting  British  Assoc.  Adv. 
Sci.,  London,  1841,  pp.  5-7. 


DEVITRIFICATION  OF  GLASS.  249 

(3)  Pressure  produces  a  state  of  unequal  strain,  and  hence  is  favorable 
to  devitrification. 

(4)  Mass-mechanical    action  not  only  produces    a  state  of  unequal 
strain  in  minerals,  but  fractures  the  material,  and  this  gives  a  large  surface 
of  action  for  the  solutions.     It  is  therefore  clear  that  mass-mechanical  action 
is  very  favorable  to  devitrification. 

(5)  Experiments  in  the  laboratory  show  that  if  glass  be  raised  to  a 
temperature    short    of  fusion    the   tendency    to    devitrification    is   greatly 
promoted.     It  is  therefore  certain  that  conditions  of  dry  heat  after  solidifi- 
cation are  favorable  to   devitrification.     As  glass  occurs  in  considerable 
bodies  in  a  state  of  nature,  it  must  for  a  long  time,  perhaps  hundreds  ot 
thousands  of  years,  have  a  high  temperature  due  to  the  residual  heat  of 
the  mag-ma,  and  only   very   gradually  assumes  the  normal    temperature 
corresponding  with  its  depth  of  burial.     It  is  rather  probable  that  micro- 
lites  and   crystallites,  which  so  frequently  occur   in    glass,  largely  form 
during  this  process  of  cooling  after  solidification. 

(6)  While  devitrification  of  glass  may  occur  without  the  presence  of 
abundant  water,  it  is  probably  rare  indeed  that  in  nature  the  process  occurs 
without  the  presence  of  some  moisture,  and  in  general  moisture  is  a  very 
important  factor  favorable  to  devitrification. 

It  is  therefore  clear  that  each  of  the  above  factors  may  give  a  condi- 
tion favorable  to  devitrification,  but  in  general  actual  devitrification  is 
due  to  a  combination  of  two  or  more  of  them. 

Devitrification  in  the  two  zones. — In  the  zone  of  katamorphisiii  under  ordinary 
conditions  it  is  probable  that  the  devitrification  occurs  somewhat  slowly. 
But  in  areas  of  regional  volcanism,  and  often  in  those  of  local  volcanism, 
the  lava  flows  follow  one  another  in  such  rapid  'succession  that  beds  are 
piled  up  so  deep  that  the  water  is  held  at  a  high  temperature.  By  complex 
intrusion  the  entire  mass  of  a  cooled  glass  may  again  be  raised  to  a  high 
temperature.  Orogenic  movement  if  severe  may  produce  a  high  tempera- 
ture. Under  any  of  these  circumstances  the  conditions  are  furnished  for 
the  complete  and  rapid  devitrification  of  the  glass. 

The  nature  of  the  devitrification  is  certainly  different  in  the  belt  of 
weathering  and  the  belt  of  cementation,  although  available  descriptions  do 
not  furnish  data  for  accurate  statements  as  to  the  differences.  But  it  is 
certain  that  in  the  belt  of  weathering  the  sevei-al  changes  are  along  the 


250  A  TREATISE  ON  METAMORPHISM. 

lines  given  on  pages  506-527  for  that  belt,  finally  resulting  in  the  oblitera- 
tion of  textures  and  structures  and  producing  an  incoherent  rock. 

In  the  belt  of  cementation  ordinarily  the  alterations  do  not  result  in 
the  obliteration  of  the  original  textures  and  structures  of  the  glasses.  This 
is  sufficiently  evident  where  the  alterations  occur  under  mass-static  condi- 
tions, and  even  where  mass-mechanical  conditions  prevail.  The  glass  is 
simply  fractured,  as  explained  on  pages  601-602,  and  the  individual  blocks 
are  altered  by  metasomatism  under  mass-static  conditions. 

So  far  as  we  know,  glasses  originally  form  only  in  the  zone  ot 
katamorphism,  and  mainly  at  or  near  the  surface.  Therefore  a  glass  can 
get  into  the  zone  of  anamorphism  only  by  being  buried  under  succeeding 
lava  flows  or  tuffs  or  under  sedimentar3T  rocks.  Hence,  before  glass 
reaches  the  lower  zone,  it  must  have  been  subjected  for  a  long  time  to 
devitrification  in  the  belt  of  cementation,  and  the  question  arises  whether 
or  not  a  glass  would  not  be  completely  devitrified  before  it  becomes 
sufficiently  deeply  buried  to  reach  the  zone  of  anamorphism.  However,  if 
glass  ever  does  reach  the  lower  zone,  it  is  certain  that  its  devitrification  will 
take  place  rapidly  under  either  mass-static  or  mass-mechanical  conditions. 
The  rocks  in  this  lower  zone  are  everywhere  at  temperatures  exceeding 
100°  C;  they  contain  water;  hence,  even  under  conditions  of  absolute 
quiescence,  it  is  certain  that  glass  could  not  long  exist.  The  crystallization 
would  be  even  more  rapid  under  mass-mechanical  conditions. 

In  so  far  as  the  glass  had  devitrified  in  the  zone  of  katamorphism,  and 
had  produced  minerals  characteristic  of  that  zone,  in  the  lower  zone  these 
minerals  would  be  recrystallized  and  minerals  formed  characteristic  of  the 
latter  zone.  If  mass-static  conditions  prevail  this  recrystallization  may  take 
place  without  obliterating  previous  textures  and  structures.  However,  if 
recrystallization  takes  place  under  conditions  of  mashing,  the  original 
textures  and  structures  are  lost,  and  minerals  are  produced  of  such  kinds 
and  proportions  as  correspond  with  the  composition  of  the  glass.  More- 
over, when  the  glass  passes  into  the  zone  of  anamorphism,  textures  and 
structures  may  be  formed  characteristic  of  the  slates,  schists,  and  gneisses. 
When  such  alteration  is  complete  it  is  often  impracticable  to  determine 
whether  the  rock  was  originally  glass  or  not.  There  can  be  little  doubt 
that  many  of  the  finer-grained  schists  are  derived  from  rocks  which  were 


DEVITRIFICATION  OF  GLASS.  251 

originally  partly  or  wholly  glassy.  For  instance,  the  Berlin  gneiss  of 
central  Wisconsin  is  in  chemical  composition  the  same  as  that  of  various 
associated  aporhyolites.  The  aporhyolites  show  that  they  were  originally 
glasses  by  retaining  the  characteristic  textures  of  glass.  The  Berlin  gneiss 
which  was  altered  under  conditions  of  mashing  in  the  deep-seated  zone  is 
entirely  devoid  of  any  .structure  which  can  be  attributed  to  glass,  and  one 
can  not  be  certain  that  it  did  originally  have  a  glassy  base,  although  this 
seems  probable. 

Minerals  produced. — The  minerals  which  are  produced  by  the  alterations  of 
glass  are  very  numerous.  It  has  already  been  noted  that  glasses  form 
from  the  most  acid  magmas,  and  also  from  those  which  are  intermediate  or 
basic  in  character.  Furthermore,  it  has  been  seen  that  glass  is  devitrified 
in  both  the  upper  and  the  lower  physical-chemical  zones,  and  in  the  upper 
zone  both  in  the  belt  of  weathering  and  in  that  of  cementation.  In  each  of 
these  zones  and  belts  minerals  form  from  the  glass  which  are  characteristic 
of  them.  It  is  plain  from  the  foregoing  that  every  mineral  which  may  be 
a  metamorphic  product  of  an  igneous  rock  of  any  kind  may  result  from  the 
devitrification  of  glasses  of  different  kinds  under  the  different  conditions 
which  obtain  in  the  zones  and  belts  of  alteration. 

Heat  and  volume  relations. — The  devitrification  of  glass  is  a  process  which 
probably  results  in  the  liberation  of  heat.  This  is  certainly  true  for  the 
zone  of  katamorphism,  where  oxidation,  hydration,  and  carbonation  take 
place.  As  to  the  volume  relations  of  the  change,  the  devitrification  itself  by 
means  of  which  the  substance  passes  from  an  amorphous  to  a  crystalline 
condition  would  decrease  the  volume,  provided  there  were  no  additions  of 
other  compounds.  But  where  devitrification  is  accompanied  by  oxidation, 
carbonation,  and  hydration  there  are  considerable  additions  of  material. 
Therefore,  in  the  belt  of  cementation  there  can  be  little  doubt  that 
expansion  of  volume  is  the  rule  where  glasses  are  devitrified;  but  in  the 
belt  of  weathering,  where  solution  is  prominent,  doubtless  there  is  diminution 
in  volume  with  glass  as  with  other  compounds.  In  the  zone  of  anamorphism 
devitrification  takes  place  with  decrease  of  volume,  the  reactions  being 
controlled  by  pressure.  Whether  heat  be  liberated  or  absorbed  in  the  zone 
of  anamorphism  doubtless  depends  in  large  measure  upon  how  far  the 
reactions  of  the  zone  of  katamorphism  have  taken  place  during  the  time 


252  A  TREATISE  ON  METAMORPHISM. 

the  glass  was  passing  through  that  zone.  If  these  had  gone  far,  the  undoing 
of  the  oxidation,  hydration,  and  carbonation  would  probably  absorb  heat. 
But  if  the  glass  reached  the  zone  of  anamorphism  in  an  anhydrous  condition, 
the  crystallization,  producing  a  decrease  in  volume,  would  liberate  heat. 
Thus  no  general  statement  can  be  made  as  to  the  heat  reaction  in  the  zone 
of  anamorphism. 

FELDSPAR   OROUP. 

The  minerals  of  the  feldspar  group  are  the  most  abundant  of  the 
silicates.  According  to  Clarke,"  the  feldspars  comprise  60  per  cent  of  the 
minerals  of  the  lithosphere.  The  feldspars  include  minerals  of  two  classes 
of  symmetry,  monoclinic  or  pseudomonoclinic,  and  triclinic.  Those  of  the 
first  class  comprise  orthoclase,  microcline,  and  anorthoclase ;  those  of  the 
second  class  include  albite,  oligoclase,  andesine,  labradorite,  bytownite, 
and  anorthite. 

In  chemical  composition  the  feldspars  vary  from  orthosilicates,  through 
metasilicates,  to  polysilicates.  The  readiness  of  decomposition  is  indirectly 
proportional  to  the  acidity,  the  orthosilicates  being  the  most  easily  decom- 
posed, and  the  polysilicates  being  the  most  difficult  to  decompose. 

The  more  frequent  alterations  of  the  monoclinic  feldspars  and  of  the 
polysilicate  plagioclase  feldspars  are  to  mica,  especially  muscovite,  and  to 
hydrated  silicate  of  aluminum,  especially  kaolin.  In  this  alteration  there 
is  simultaneous  liberation  of  silica,  which  may  separate  as  quartz.  Very 
frequently  also  gibbsite  is  formed  at  the  same  time.  Where  the  mica  biotite 
is  produced  it  is  necessary  that  iron  and  magnesium  shall  be  added.  The 
most  common  alterations  of  the  orthosilicate  plagioclase  feldspars  are  to 
zeolites,  epidote,  and  zoisite,  frequently  with  the  simultaneous  formation  of 
another  plagioclase  and  chlorite.  Where  epidote  is  produced  it  is  necessary 
that  iron  be  added  from  some  other  source;  where  chlorite  is  produced  it  is 
necessary  that  magnesium  and  iron  be  added  from  some  other  source. 
All  the  important  minerals  produced  by  the  alterations  of  the  feldspars, 
with  the  exception  of  quartz  and  plagioclase,  are  hydrated,  though  in 
varying  degrees;  hence,  in  general,  water  is  added  during  the  alteration  of 
the  feldspars.  From  the  intermediate  plagioclases  there  may  be  produced 
any  of  the  foregoing  minerals 

"Clarke,  F.  W.,  Analyses  of  rocks,  laboratory  of  the  TJ.  S.  Geol.  Survey,  1880-1899:  Bull.  U.  S. 
i ;..,,!.  Survey  No.  168,  1900,  p.  16. 


ORTHOCLASE  AND  MICROCLINE.  253 

MOXOCL1SIC  OK  I'SKI'DOMONOCLIXK'. 

HIM  HIM  I.  isl..    1111  1:01  I.I  \l.  AND  AXOBTHOCLASE. 
Orthoclase :   • 

KA18i,O8. 

Monoclinic. 

Sp.  gr.  2.57. 
Microcline: 

KAlSi308. 

Triclinic. 

Sp.  gr.  2.54-2.57. 
Anorihodase: 

™NaA],Si3O8.nKAl,,Si3O8.     (Na-silicate:K-si1ioate::  2:1  or  3:1,  usually.) 

Pseudornonoclinic  or  triclinic. 

Sp.  gr.  2.57-2.60. 

ORTHOCLASE    ANJ)    MICROCLINK. 

occurrence. — Orthoclase  and  microcline  have  a  very  widespread  occur- 
rence as  chief  pyrogenic  constituents.  The  minerals  also  are  allogenic 
constituents  of  the  clastic  rocks.  They  further  have  a  very  widespread 
occurrence  in  the  metamorphic  rocks,  being  chief  constituents  both  as 
allogenic  and  as  autogenic  constituents  of  the  schists  and  gneisses  of  both 
aqueous  and  igneous  origin.  In  the  development  of  the  feldspars  as 
autogenic  constituents  it  is  usually  necessary  that  two  or  more  minerals 
unite,  except  in  the  case  of  the  derivation  of  the  acid  feldspars  from  the 
more  basic  ones  or  from  leucite.  As  a  metamorphic  mineral  orthoclase  is 
derived  from  analcite,  heulandite,  leucite,  laumontite,  and  stilbite.  Micro- 
cline is  recorded  as  derived  from  spodumene. 

Alterations. — One  of  the  most  important  alterations  of  orthoclase  and 
microcline  is  to  kaolinite  (monoclinic;  sp.  gr.  2.60-2.63).  The  most  prob- 
able reaction,  for  reasons  given  below,  is  believed  to  be: 

(1)     2KAlSisO8+2H2O+CO2=H4Al2Si2O9+4SiO2+K2CO3+k. 

The  decrease  in  volume,  supposing  the  freed  silica  to  separate  as  quartz, 
and  K2CO3  dissolved,  is  12.57  per  ce.nt.  If  all  of  the  freed  silica  be  dis- 
solved, the  decrease  in  volume  would  be  54  44  per  cent.  In  calculating 
these  volume  changes  and  those  which  follow,  the  specific  gravity  of 
orthoclase  is  used. 

While  the  ordinary  alteration  of  the  potash  feldspars  to  the  kaolin 
group  is  to  kaolinite  as  indicated,  the  alteration  may  be  to  other  minerals 
of  this  group;  for  instance,  to  pyrophyllite  (monoclinic  (?);  sp.  gr.  2.8-2.9), 
halloysite  (massive;  sp.  gr.  2.1),  newtonite  (rhombohedral ;  sp.  gr.  2.37), 


L>;>4  A  TREATISE  ON  METAMORPHISM. 

cimolite  (amorphous;  sp.  gr.  2.24),  nllophane  (amorphous;  sp.  gr.  1.87),  and 
perhaps  others.  The  chief  differences  are  in  the  amounts  of  water  added 
and  the  amount  of  silica  which  separates.  Pyrophvllite  (H2Al2(SiOa)4) 
differs  from  kaolinite  in  that  less  silica  is  removed  and  less  basic  water  is 
added;  it  therefore  might  be  considered  as  an  intermediate  stage  in  the 
alteration.  Halloysite  (H4Al2Si2O9  Aq.)  differs  from  kaolinite  only  in  having 
water  of  hydration.  Newtonite  (HgAl2Si2On.Aq.)  differs  from  kaolinite  in 
containing  twice  as  much  basic  water  as  that  mineral,  and  in  being  hydrated. 
Cimolite  (H6Al4(SiO3)9.3H2O)  differs  from  kaolinite  in  containing  more 
silica,  more  basic  water,  and  water  of  hydration.  Allophane  (Al2Si05.5H2O) 
differs  from  kaolinite  in  containing  less  silica  and  much  water.  It  would 
be  easily  possible  to  formulate  equations  along  the  line  of  that  given  for 
kaolinite  for  each  of  these  minerals  and  to  calculate  the  volume  relations. 
However,  this  hardly  seems  necessary  since  these  minerals  as  secondary 
products  to  orthoclase  and  microcline  appear  to  be  Very  subordinate  in 
amount. 

Another  alteration  of  orthoclase  and  microcline  of  some  little  impor- 
tance is  into  gibbsite  (monoclinic;  sp.  gr.  2.3-2.4).  The  reaction  is: 

(2)  2KAlSisO84-3H2O+CO2=2Al(OH)8+6Si02+K2CO3+k. 

The  decrease  in  volume  of  the  gibbsite  and  quartz  as  compared  with  the 
orthoclase  is  6.61  per  cent. 

Another  of  the  very  important  alterations  of  orthoclase  and  microcline 
is  to  muscovite  (monoclinic;  sp.  gr.  2.76-3.0)  and  quartz  (rhomboheclral; 
sp.  gr.  2.653-2.654).  The  reaction  is: 

(3)  SKAlSijOj+HjO+CO^KHjAlsSiAi+SSiOj+KjCOs+k. 

Provided  the  silica  separates  as  quartz  and  the  potassium  unites  with 
carbonic  acid  and  the  potassium  carbonate  be  removed  in  solution,  the 
decrease  in  volume  is  15.58  per  cent. 

While  this  reaction  may  take  place  under  exceptional  conditions,  it  is 
believed,  as  explained  below,  that  where  muscovite  forms  from  orthoclase 
one  of  the  rich  aluminous  minerals  often  unites  with  the  orthoclase  to 
produce  the  mica.  Supposing  the  aluminous  mineral  to  be  gibbsite,  the 
reaction  is : 

(4)  KAlSi,08+2Al(OH)s=KH2A1sSi,01.!+2H20-k. 

The  decrease  in  volume  of  the  muscovite  as  compared  with  the  orthoclase 
and  gibbsite  is  20.81  per  cent. 


'ALTERATIONS  OF  ORTHOCLASE  AND  MICROCLINE.  255 

The  alteration  may  be  to  hydro-muscovite  or  damourite  (monoclinic; 
sp.  gr.  2.76-3.00).  This  is  believed  by  most  mineralogists  to  differ  from 
muscovite  only  in  containing  more  water,  but  Dana  states  that  a  greater 
content  of  water  in  damourite  than  that  contained  by  ordinary  muscovite 
is  not  necessary. 

From  orthoclase  and  microcline,  with  the  addition  of  magnesium  and 
iron  compounds,  biotite  (monoclinic;  sp.  gr.  2.7-3.1)  may  be  formed.  If 
the  hydrogen  and  potassium  be  supposed  to  be  present  in  equal  proportions 
and  the  same  supposition  be  made  with  reference  to  magnesium  and  iron, 
and  the  latter  elements  are  supposed  to  be  present  as  carbonates,  the 
reaction  may  be  as  follows : 

(5)  4KAlSi3O8+2MgCO3+2FeCO3+H2O== 

2H  KMgFe  A  ]2Si3O12  •+  5SiO2 + K2SiO3 +4CO2+k. 

The  decrease  in  volume  of  the  feldspar,  magnesium  carbonate,  and  iron 
carbonate  in  passing  into  the  biotite  and  quartz  is  22.64  per  cent.  But,  as 
with  muscovite,  the  more  frequent  reaction  probably  involves  gibbsite, 
thus: 

(6)  KAlSi308fMgCO3+FeCO3+Al(OH)3=HKMgFeAl2Si3O12+H2O+2CO2+k. 

This  greatly  simplifies  the  equation.  The  decrease  in  volume  of  the  bio- 
tite as  compared  with  the  compounds  from  which  it  is  derived  is  22.33 
per  cent. 

Orthoclase  and  microcline  are  said  also  to  alter  to  epidote  (monoclinic; 
sp.  gr.  3.25-3.50);  but  if  this  be  so  calcium  and  iron  must  be  introduced. 
The  forms  in  which  these  compounds  are  present  during  the  alteration  are 
doubtless  variable.  If  they  be  assumed  to  be  present  as  calcium  carbonate 
and  iron  sesquioxide,  the  reaction  might  be  as  follows: 

(7)  4KAlSi3O8+Fe203+4CaCO3+H.(O=2HCa2Al.!FeSi30,3+6SiO2+2K.,CO3+2CO2+k. 

Supposing  the  Al  is  to  the  Fe  as  2  is  to  1,  the  decrease  in  volume  of  the 
epidote  and  quartz  as  compared  with  the  feldspar,  calcite,  and  iron  oxide 
together  is  33.73  per  cent.  However  it  is  so  uncertain  as  to  the  forms  of 
the  accessory  compounds,  both  before  and  after  reaction,  that  it  is  impos- 
sible to  make  a  definite  statement  as  to  the  volume  relations. 

The  alteration  of  orthoclase  and  microcline  to  minerals  of  the  kaolin 
group  and  to  gibbsite  occurs  in  the  zone  of  katamorphism.  The  process 
takes  place  on  the  most  extensive  scale  in  the  belt  of  weathering,  especially 


256  A  TREATISE  ON  METAMORPHISM. 

in  the  soil  horizon.  Wherever  the  feldspathic  rocks  are  exposed  to  atmos- 
pheric agencies  this  change  steadily  goes  on,  though  not  so  rapidly  as  with 
the  orthosilicate  feldspars.  (See  p.  519.)  But  wherever  the  potash  feld- 
spars have  been  very  long  exposed  to  the  weathering  agencies  they  have 
been  partly  or  wholly  decomposed,  and  in  some  places  to  a  depth  of  several 
hundred  feet.  The  change  is  one  of  the  most  important  of  all  those  which 
affect  rocks.  It  is  partly  because  the  alterations  take  place  near  the 
surface,  where  carbon  dioxide  is  abundant,  that  it  is  believed  that  the  freed 
alkali  largely  unites  with  carbon  dioxide,  as  given  in  the  reaction.  The 
silica  freed  in  the  belt  of  weathering  is  in  part  undoubtedly  taken  into 
solution  as  colloidal  silicic  acid  and  carried  downward  to  the  belt  of 
cementation.  Indeed,  the  silica  for  the  process  of  silicification  in  this  belt, 
which,  as  explained  on  page  480,  is  derived  from  the  decomposition  of  the 
silicates,  probably  in  good  part  comes  from  the  alteration  of  the  feldspars. 
Under  the  same  conditions  in  which  a  part  of  the  feldspar  breaks  up  into 
kaolinite  another  part  of  the  feldspar  may  produce  gibbsite,  quartz,  and 
potassium  carbonate.  The  potassium  carbonate  liberated  at  the  time  of 
the  formation  of  the  kaolinite  and  gibbsite  is  largely  dissolved  and  trans- 
ported elsewhere,  although  the  soluble  potassium  compounds  are  often  held 
in  the  soil  to  a  considerable  extent.  (See  pp.  498,  541—543.) 

The  alteration  of  orthoclase  and  microcline  to  minerals  of  the  kaolin 
group  and  to  gibbsite  is  not,  however,  confined  to  the  belt  of  weathering. 
It  takes  place  on  an  important  scale  in  the  belt  of  cementation,  though  not 
on  a  scale  comparable  to  that  in  the  belt  above.  So  far  as  known,  kaolin- 
ization  is  not  a  reaction  which  occurs  in  the  zone  of  anamorphism;  at  least, 
if  it  does  there  take  place  it  is  a  very  subordinate  phenomenon.  As  seen 
above,  the  reaction  is  one  taking  place  with  liberation  of  heat  and  fre- 
quently with  decrease  of  volume,  since  much  and  perhaps  the  most  of  the 
freed  silica  is  taken  away  in  solution.  The  heat  reaction  controls,  and 
hence  the  change  is  under  the  rules  of  the  upper  physical-chemical  zone. 

The  alteration  of  orthoclase  and  microcliue  to  mica  occurs  in  rocks 
which  have  been  somewhat  deeply  buried,  and  the  change  has  been  noted 
in  connection  with  both  mass-static  and  mass-mechanical  action.  Under 
either  of  these  conditions  the  alteration  may  be  nearly  or  quite  complete. 
But  it  has  taken  place  on  the  most  extensive,  scale  in  connection  with  mass- 
mechanical  action,  where  the  secondary  structures,  such  as  cleavage,  are 


OCCURRENCE  OF  ANORTHOCLASE.  257 

produced,  and  therefore  in  the  belt  of  rock  flowage.  In  the  formation  of 
muscovite  and  quartz  from  feldspar  by  equation  (3),  as  the  specific  gravity 
of  the  separated  quartz  is  somewhat  greater  than  that  of  the  original 
feldspar,  and  that  of  the  muscovite  is  considerably  greater  than  that  of  the 
feldspar,  the  condensation  in  volume  above  calculated  is  accounted  for, 
although  the  alteration  is  one  involving  hydration  and  possibly  carbonation. 
In  the  zone  of  anamorphism  the  wateradded  is  doubtless  largely  derived  from 
other  minerals,  as  this  is  a  belt  of  dehydration,  and  destruction  of  previous 
minerals  containing  hydroxides.  This  passage  from  one  mineral  to  another 
would  involve  no  increase  in  the  total  volume,  the  controlling  consideration. 
The  most  doubtful  point  concerning  equation  (3)  is  the  carbonation  of  the 
potassium.  It  might  be  supposed  that  the  potassium  unites  with  a  part  of 
the  freed  silica  and  with  other  elements  to  form  potassium  minerals.  But  it 
is  not  easy  to  suggest  such  minerals,  as  leucite  is  not  recorded  as  a  meta- 
morphic  mineral.  The  more  probable  solution  of  the  problem  is  that 
potassium  and  a  portion  of  the  silica  unite  with  the  alumina  of  the  gibbsite 
or  some  other  minerals  and  produce  one  molecule  of  mica  from  one  of 
orthoclase,  as  suggested  in  equation  (4).  This  suggestion  is  rendered 
especially  plausible  for  the  slates,  schists,  and  gneisses  derived  from  sedi- 
ments, for  such  rocks  usually  contain  residual  orthoclase  and  also  aluminum 
hydroxide.  (See  pp.  232,  235,  898-900.)  The  reaction  of  equation  (4)  pro- 
duces great  decrease  in  volume,  is  one  of  dehydration,  and  thus  absorbs  heat; 
it  is  therefore  a  perfect  example  of  the  rules  of  the  zone  of  anamorphism. 
The  same  remarks  are  applicable  to  equations  (5)  and  (6),  respectively,  for 
the  production  of  biotite,  as  to  (3)  and  (4)  for  the  formation  of  muscovite, 
with  the  addition  that  the  development  of  biotite  involves  silication  and 
decarbonation,  and  therefore  still  better  than  muscovite  illustrates  the 
reactions  of  the  zone  of  anamorphism. 

The  physical-chemical  principles  for  the  alteration  of  orthoclase  and 
microcline  to  epidote  are  the  same  as  for  the  alterations  of  the  more  basic 
feldspars  to  epidote.  As  the  process  occurs  much  more  extensively  in  con- 
nection with  the  latter  minerals,  it  is  discussed  under  the  basic  plagioclases. 
(See  pp.  263-264.) 


ANORTHOCLASE. 


occurrence — This  mineral  is  subordinate  in  quantity  to  orthoclase  and 
microcline.     It  occurs  in  both  deep-seated  and  effusive  igneous  rocks;  in 
MON  XLVII — 04 17 


258  A  TREATISE  ON  METAMORPHISM. 

the  latter,  chiefly  in  the  andesitic  lavas.  As  an  allogenic  mineral  it  also 
is  found  in  the  sedimentary  rocks.  Whether  it  occurs  as  an  autogenic 
mineral  in  the  inetamorphic  rocks  has  not  been  determined. 

Alterations. — Both  orthoclase  and  microclme  contain  some  sodium.  When 
the  sodium  becomes  important  the  mineral  is  anorthoclase.  It  naturally 
follows  from  this  fact  that  the  alterations  of  anorthoclase  are  in  all  respects 
like  those  of  orthoclase  and  microclme,  with  the  exception  that  the  freed 
alkalies  are  in  good  part  sodium.  The  reactions  are  analogous  to  those 
already  given  for  orthoclase,  but  with  the  muscovite  or  biotite  the  soda-mica 
paragonite  (monoclinic;  sp.  gr.  2  84)  is  formed.  Supposing  the  sodium 
silicate  is  to  the  potassium  silicate  as  2  to  1,  the  more  important  reactions 
may  be  written  as  follows: 

(1)  2(2NaAlSis08.KAlSi308)+6H/J+3C02=3H4Al2SiA+12SiO.!+K2OOs+2NaiC08+k. 

(2)  2(2NaAlSi3O8.KAlSi3O8)+9H2O+3CO8=6Al(OH)3+18SiO2+K2COs+2]STa.1CO8+k. 

(3)  2NaAlSi3O8.KAlSisO8+6Al(OH)3=KH2Al3Si3O12+2NaH2Al3SisO12+6H20--k. 

(4)  2NaAlSi3O8.KAlSi3O8+MgCO3+FeCO,+5Al(OH)3= 

HKMgFeAl2Si3O,2+2NaH2Al3Si3O12+5H2O+2CO2— k. 

Supposing  the  sodium  silicate  is  to  the  potassium  silicate  as  3  to  1,  we  have: 

(5)  2(3NaAlSi308.KAlSi3OB)+2F6,03+8CaCO3+2H2O= 

4HCa2Al2FeSi3O13412SiO2+K2C03+3Na2CO3+4COj+k. 

The  equations  corresponding  to  (3)  and  (5)  under  orthoclase  and 
microcline  are  not  written,  since  their  occurrence  is  very  doubtful.  The 
decrease  in  volume  of  the  kaolinite  and  quartz  as  compared  with  the 
anorthoclase,  equation  (1),  is»  9.56  per  cent,  or  of  the  kaolinite  alone  is  52.19 
per  cent.  The  decrease  in  volume  of  the  gibbsite  and  quartz  as  compared 
with  the  anorthoclase,  equation  (2),  is  3.30  per  cent,  or  of  the  gibbsite  alone 
is  68.02  per  cent.  The  decrease  in  volume  of  the  muscovite  and  paragonite, 
as  compared  with  the  anorthoclase  and  gibbsite,  equation  (3),  is  20.04  per 
cent,  The  decrease  in  volume  of  the  biotite  and  paragonite,  as  compared 
with  the  anorthoclase  and  gibbsite,  equation  (4),  is  10.91  per  cent.  The 
decrease  in  volume  of  the  epidote  and  quartz,  as  compared  with  the  anortho- 
clase, hematite,  and  calcite,  equation  (5),  is  28.30  per  cent.  Equations  cor- 
responding with  the  above  and  the  volume  relations  can  be  easily  worked 
out  along  analogous  lines  for  other  ratios  of  the  sodium-bearing  and  potas- 
sium-bearing silicates,  but  the  general  results  would  be  the  same,  so  this  is 
hardly  worth  the  while. 

The    geological   positions  and   physical  conditions  under  which   the 


THE  PLAGIOCLASE  FELDSPARS.  259 

changes  take  place  are  identical  with  the  corresponding  changes  of  ortho- 
clase  and  microcline — i.  e.,  alterations  represented  by  equations  (1)  and  (2) 
take  place  in  the  zone  of  katamorphism,  and  especially  the  belt  of  weathering. 
Alterations  (3)  and  (4)  occur  in  the  zone  of  anamorphism,  and  that  of 
equation  (5)  is  known  for  the  belt  of  cementation.  One  general  point  is 
clear  from  the  above,  that  in  the  anorthoclase  rocks  we  have  a  source  for 
paragonite  in  the  paragonite-schists  and  paragonite-gneisses. 

TRICLINIC. 

The  plagioclase  feldspars  are  a  group  of  triclinic  feldspars  which  range 
from  sodium-aluminum  silicate  to  calcium-aluminum  silicate.  The  former 
is  a  polysilicate  and  the  latter  an  orthosilicate,  hence  there  is.  great  variation 
both  as  to  composition  and  as  to  acidity.  The  names,  compositions,  and 
specific  gravities  of  the  species,  as  given  by  Tschermak  and  Dana,  are  as 
follows: 

U.ltn  I,.  OI.K.OI  HM  .  AXDESIXE,  LABBADOBITE,  BYTOWXITE,  AXD  AXOBTHITE. 

Attnte: 

NaAlSi,08. 

Triclinic. 

Sp.  gr.  2.62-2.65. 
Oligoclase: 

Ab  to  AbsAiij. 

Triclinic. 

Sp.  gr.  2.65-2.67. 
Andesine: 

AbsAn,  to  A^An,. 

Triclinic. 

Sp.  gr.  2.68-2.69. 
Labradorite: 

Ab,Ani  to  Ab,An3. 
.  Triclinic. 

Sp.  gr.  2.70-2.72. 
Bylcnvnite: 

AbjAnj  to  An. 

Triclinic. 

Sp.  gr.  2.72-2.74. 
Anorthite: 

CaAl2SiA- 

Triclinic. 

Sp.  gr.  2.74-2.76. 

occurrence. — The  plagioclases  are  probably  the  most  important  rock- 
making  constituents,  being  approached  in  abundance  only  by  the  orthoclase 
feldspars  and  by  quartz.  The  plagioclases  are  present  as  pyrogenic  con- 
stituents in  the  great  majority  of  igneous  rocks.  They  also  occur  very 


260  A  TREATISE  ON  METAMOEPHISM. 

» 

abundantly  as  allogenic  constituents  in  the  sedimentary  rocks.  In  such 
rocks  the  more  siliceous  plagioclases  are  more  plentiful  than  the  less  siliceous 
plagioclases,  because  of  the  more  ready  decomposition  of  the  latter.  The 
plagioclases  develop  abundantly  as  autogenic  constituents  in  the  metamor- 
phic  rocks  of  both  sedimentary  and  igneous  origin.  The  plagioclase  albite 
is  recorded  as  being  derived  from  analcite,  heulandite,  laumontite,  plagio 
clases  (with  orthoclase),  sodalite,  spodumene,  and  stilbite.  The  plagioclase 
anorthite  is  not  recorded  as  being  derived  from  other  minerals.  But  it  is 
seen  in  the  zone  of  katamorphism  that  anorthite  passes  into  various  zeolites 
by  simple  hydration.  It  can  hardly  be  doubted  that  when  such  zeolites 
pass  into  the  zone  of  anamorphism  by  dehydration  they  are  sources  of  anor- 
thite. Doubtless  also  anorthite  is  produced  in  different  ways  from  the  com- 
binations of  various  minerals,  just  as  it  passes  into  different  combinations  of 
minerals.  The  intermediate  feldspars,  which  are  intermolecular  mixtures  of 
albite  and  anorthite,  may  be  derived  from  any  of  the  minerals  from  which 
albite  and  anorthite  are  formed. 

Alterations. — In  treating  the  alterations  of  the  plagioclases  the  only  prac- 
ticable plan  is  to  calculate  equations  and  volume  reactions  separately  for 
albite  and  for  anorthite.  For  any  of  the  intermediate  feldspars  the  corre- 
sponding equations  may  be  written  by  multiplying  the  albite  and  anorthite 
equations  by  the  number  of  molecules  of  these  compounds,  respectively, 
and  adding  the  products.  However,  the  alterations  of  the  plagioclases  are 
so  complicated  that  I  have  not  been  able  to  make  the  treatment  more  than 
very  partial. 

The  species  belonging  to  the  more  siliceous  half  of  the  plagioclase  feld- 
spars— i.  e.,  albite,  oligoclase,  and  audesine — frequently  undergo. alterations 
similar  to  those  of  the  monoclinic  feldspars,  producing  kaolin  (monoclinic; 
sp.  gr.  2.615),  gibbsite  (monoclinic;  sp.  gr.  2.35),  and  quartz  (rhombohe- 
dral;  sp.  gr.  2.6535).  These  alterations  may  be  considered  as  coming 
from  the  albite  molecule. 

But  the  more  common  alterations  of  the  plagioclases  are  into  the  zeo- 
lites, epidote  (monoclinic;  sp.gr.3.38),  quartz,  the  scapolites,  and  paragonite 
(monoclinic;  sp.  gr.  2.84),  and  the  less  siliceous  feldspars  into  more  siliceous 
plagioclase  feldspars.  The  plagioclases  are  also  recorded  as  altering  into 
prehnite  (orthorhombic;  sp.  gr.  2.875)  and  albite.  By  pyrochemical 
methods  plagioclase  and  sodium  carbonate  at  220°  C.  produce  the  zeolite 
analcite  (isometric;  sp.  gr.  2.255),  and  this  process  is  more  rapid  in  propor- 


ALTERATIONS  OF  PLAGIOCLASE  FELDSPARS.       261 

tion  as  the  feldspars  are  less  siliceous.  The  alteration  of  a  given  feldspar 
may  be  into  two  or  more  of  the  above  minerals.  Doubtless  often  orthoclase 
and  plagioclase  together  pass  into  other  minerals.  One  such  reaction  has 
been  considered  by  Becke,  and  is  given  below. 

The  less  siliceous  plagioclases,  labradorite,  bytownite,  and  anorthite, 
alter  rarely  to  kaolin  alone,  but  this  mineral  may  separate  simultaneously 
with  zoisite  (orthorhombic;  sp.  gr.  3.31)  or  epidote. 

The  alteration  of  albite  to  kaolin  and  quartz,  to  gibbsite  and  quartz, 
and  of  albite  and  gibbsite  to  paragonite,  respectively,  may  be  written  as 
follows : 

(1)  2NaAlSi3O8+2H2OrCO,=H4Al2Si2O9^4SiO2+Na2CO3+k. 

(2)  2NaAlSi3O8^3H2O+C02=2[Al(OH)3]+6Si024-Na2CO3+k. 

(3)  NaAlSi308+2Al(OH)3=NaH2Al3Si3O12+2H2O+k. 

The  decrease  in  volume  in  equation  (1)  of  the  kaolin  and  quartz  is  4.89 
per  cent;  in  equation  (2)  the  increase  for  the  gibbsite  and  quartz  is  1.58  per 
cent;  in  equation  (3)  the  decrease  for  the  paragonite,  as  compared  with  the 
albite  and  gibbsite,  is  18.85  per  cent. 

Analcite  (isometric;  sp.  gr.  2722-2.29)  may  be  derived  from  albite 
according  to  the  following  reaction : 

(4)  2NaAlSi3O8+2H2O=Xa2Al2Si4O12.2H2O+2SiO2+k. 

The  increase  in  volume  is  20.82  per  cent,  supposing  the  silica  separates  as 
a  solid. 

Natrolite  (orthorhombic;  sp.  gr.  2.20-2.25)  may  also  be  derived  from 
albite  according  to  the  reaction: 

(5)  2NaAlSi3O8+2H2O  =  H4Na2Al2Si3Oi2+3SiO2+k. 

The  increase  in  volume  is  19.95  per  cent,  supposing  the  silica  separates  as 
a  solid. 

From  anorthite  a  number  of  zeolites  are  derived.  Clarke  is  one  of  the 
latest  authors  who  has  discussed  the  relations  of  the  zeolites  to  the  feldspars, 
;ind  the  chemical  alterations  given  are  obtained  mainly  from  his  paper." 
The  equations  for  the  more  commpn  varieties  may  be  written  as  follows: 

Thomsonite  (orthorhombic;  sp.  gr.  2.3-2.4)  is  derived  from  anorthite 
according  to  the  following  reaction : 

(6)  3CaAl2Si2O8+7H2O=Ca3Al6Si6O24-17H2O+k. 

The  increase  in  volume  is  34.65  per  cent. 

o  Clarke,  F.  W.,  The  constitution  of  the  silicates:  Bull.  TJ.  S.  Geol.  Survey  No.  125,  1895,  pp.  32-45. 


262  A  TREATISE  ON  METAMORPHISM. 

Gismondite  (monoclinic;  sp.  gr.  2.265)  is  derived  from  anorthite 
according  to  the  following  reaction: 

(7)  3CaAl:,SiA+12H;!0=Ca3Al6Si6021.12HJ0+k. 

The  increase  in  volume  is  52.76  per  cent. 

For  laumontite  (monoclinic;  sp.  gr.  2.305)  the  change  does  not  appear 
to  have  been  determined  with  reasonable  certainty.  It  may  be  supposed 
to  be  derived  from  anorthite  by  the  simultaneous  union  of  freed  calcium 
and  aluminum  with  other  compounds,  the  calcium  perhaps  passing  into  the 
carbonate  and  the  aluminum  into  the  hydrate.  On  this  hypothesis  the 
reaction  is: 

(8)  2CaAlsSi208+7H20+C02=H1CaAl.1Si4Ou-2H20+CaCO,,+2[A1(OH)3]+k. 

The  increase  in  volume  is  33.65  per  cent,  supposing  the  calcium  carbonate 
to  be  dissolved  and  the  aluminum  hydroxide  to  remain  as  gibbsite.  How- 
ever, Clarke  regards  laumontite  as  derived  from  equal  quantities  of  anorthite 
and  the  hypothetical  compound  trisilicic  anorthite a  (Ca3Al6(Si3O8)6). 

The  zeolite  phillipsite  (monoclinic;  sp.  gr.  2.20)  may  be  regarded  as 
derived  from  albite,  anorthite,  and  leucite,  as  follows: 

(9)  6CaAl2Si2084-4NaAlSi5O8+6KAlSi2O6+48H2O+2CO2= 

3(K2Ca2Al6Si12O36.14H2O)+2Na2CO3+4Al(OH)s+k. 

The  leucite  is  added  as  a  source  of  the  potassium.  The  increase  in  volume 
of  the  three  compounds  in  passing  into  phillipsite  is  31.98  per  cent. 

Heulandite  (epistilbite)  (monoclinic;  sp.  gr.  2.20)  and  stilbite  (mono- 
clinic;  sp.  gr.  2.1495)  are  regarded  by  Clarke  as  derived  from  the 
hypothetical  compound  trisilicic  anorthite.  Chabazite  (rhombohedral ;  gp. 
gr.  2.12)  is  regarded  by  him  as  derived  from  this  compound  and  from 
normal  anorthite.  All  four,  however,  may  be  equally  well  considered 
as  derived  from  intermediate  plagioclases  with  carbonation  of  the  excess 
of  calcium  and  hydration  of  the  excess  of  aluminum.  On  these  hypotheses 
the  reactions  for  the  four  may  be  written  as  follows: 

(10)  4NaAlSisO8+3CaAl2Si2O8+21H2O+2CO2= 

3(H4CaAl2Si6Oie.3H2O)+2Na2C0344[Al(OH)s]+k. 

(11)  4NaAlSi308+3CaAl2Si2O8+24H2O+2CO2= 

Oa3Al6(Si3O8)6.18H2O+2Na2CO3+4Al(OH)s+k. 

(12)  6NaAlSi8O8+6CaAl,Si2O8+3C02+45H2O= 

2[Ca,Al6(Si04)s(Sis08)8.18H!0]+3NaJCp5+6[Al(OH)s]+6SiOI-l-k. 

"Clarke,  F.  W.,  The  constitution  of  the  silicates:  Bull.  U.  S.  Geol.  Survey  No.  125,  1895,  p.  42. 


ALTERATIONS  OF  PLAGIOCLASE  FELDSPARS.       263 

I 

Supposing  the  sodium  carbonate  is  dissolved  and  the  other  compounds 
are  solids,  the  increase  in  volume  for  (10)  is  37.14  per  cent,  for  (11)  is 
43.50  per  cent,  and  for  (12)  is  46.76  per  cent. 

Scolecite  (monoclinic;  sp.  gr.  2.16-2.40)  may  be  derived  from  anorthite, 
according  to  the  following  reaction: 

(13)  3CaAl2Si2O8+9H2O+CO2=2CaAl2Si3OI0.3H2O+2Al(OH)3+CaCOs+k. 

The  increase  in  volume  is  35.23  per  cent,  provided  the  gibbsite  separates 
as  a  solid  and  the  CaC03  is  dissolved. 

Mesolite  (monoclinic  or  triclinic;  sp.  gr.  2.29),  according  to  Clarke,0 
is  an  isomorphous  mixture  of  equal  quantities  of  natrolite  and  scolecite; 
therefore  the  reaction  for  this  compound  may  be  expressed  by  the  following: 

(14)  4NaAlSi3O8+3CaAl2Si2O8-!-13H20+CO2= 

2(H8Na2CaAl4Si6O24.H2O)+6Si02+2Al(OH)3+CaC03+k. 

The  expansion  in  volume  is  24.96  per  cent,  provided  the  silica  and  gibbsite 
separate  as  solids  and  the  CaCO3  is  carried  away  in  solution.  If  all  products 
are  solid  the  increase  in  the  volume  is  30.19  per  cent. 

Turning  now  from  the  zeolites  to  other  minerals,  the  plagioclases  are 
recorded  as  altering  into  prehnite  (orthorhombic;  sp.  gr.  2.875).  Since 
albite  is  recorded  as  simultaneously  separating,  the  most  probable  reaction 
is  by  hydratiou  of  the  anorthite  molecule. 

(15)  4CaAl2Si2O8+8H2O=2H2Ca2Al2Si3OI2+4Al(OH)3+2SiO2+k. 

Supposing  the  compounds  formed  to  be  solids,  the  increase  in  volume  is 
14.85  per  cent. 

Another  important  alteration  of  the  plagioclases  is  into  zoisite 
(orthorhombic;  sp.gr.  3.25-3.37)  or  epidote  (monoclinic;  sp.  gr.  3.25-3.5), 
with  the  simultaneous  formation  of  kaolinite  or  gibbsite.  The  reaction  for 
anorthite  in  the  case  of  zoisite  is  probably: 

(16)  4CaAl2Si2O8+3H20=H2Ca4Al6Si6O26+H4Al2Si2O9+k, 
or 

(17)  4CaAl2Si2O8+4H2O=H2CatAl6Si6O26+2Al(OH)3+2SiO2+k. 

The  decrease  in  volume  of  the  solids  in  (16)  is  7.77  per  cent,  and  in  (17) 
is  4.58  per  cent. 

In  the  formation  of  epidote  the  reactions  are  of  a  similar  kind,  but 
Fe2O3  replaces  some  A12O3  of  the  feldspar  molecule.  Supposing  the 

a  Clarke,  cit..  Bull.  125,  pp.  35-36. 


264  A  TREATISE  ON  METAMORPHISM. 

aluminum  is  to  the  iron  as  2  to  1,  that  the  iron  is  derived  from  hematite,  and 
that  excess  of  aluminum  separates  as  gibbsite,  the  reaction  for  anorthite  is: 

(18)  4CaAl2Si208+Fe203+6H20=H2Ca4Al4Fe2SiA6+H4Al2Si209+2Al(OH)3+k, 

or 

(19)  4CaAl2SiA+7H20+F&A=H2Ca4Al4Fe2Si6026+4Al(OH)s+2Si02+k. 

The  increase  of  the  volume  of  the  epidote,  kaolin,  and  gibbsite  as  compared 
with  the  anorthite  and  hematite,  equation  (18),  is  3.6  per  cent.  The 
increase  in  volume  of  the  epidote,  gibbsite,  and  quartz  as  compared  with 
the  anorthite  and  hematite,  equation  (19),  is  6.57  per  cent. 

The  scapolites  include  marialite  (tetragonal;  sp.  gr.  2.566),  meionite 
(tetragonal;  sp.  gr.  2.70-2.74),  and  various  isomorphous  mixtures.  (See 
pp.  311-312.)  The  alterations  of  the  plagioclases  into  these  two  minerals 
are  given.  From  these  equations  those  for  any  definite  isomorphous  mix- 
ture of  marialite  and  meionite  can  easily  be  formulated.  According  to 
Clarke,"  albite  changes  into  marialite,  and  anorthite  into  meionite.  The 
reactions  may  be  written  as  follows : 

(20)  3NaAlSi308+NaCl=Na4Al,Si9O,4Cl+k. 

Supposing  the  NaCl  to  be  in  solution,  the  increase  in  volume  of  the  solid 
compound  is  10.29  per  cent.  If  the  NaCl  be  supposed  to  be  a  solid,  the 
increase  in  volume  of  the  solid  compound  is  1.84  per  cent.  The  change 
from  anorthite  to  meionite  is  represented  by  the  following  reaction: 

(21)  3CaAl2Si,O8+CaCOa=Ca4Al6Si6O26+CO2+k. 

If  the  calcium  carbonate  be  supposed  to  be  present  as  a  solid,  the  decrease 
in  volume  is  3.78  per  cent;  if  it  be  supposed  to  be  added  in  solution,  the 
increase  in  volume  is  7.87  per  cent. 

Becke  records  the  alteration  of  orthoclase  and  plagioclase  into  albite 
(tri clinic;  sp.  gr.  2.635),  zoisite  (orthorhombic;  sp.  gr.  3.31),  muscovite 
(monoclinic;  sp.  gr.  2.88),  and  quartz  (rhombohedral;  sp.  gr.  2.6535). 
His  equation  for  this  reaction  is  as  follows: b 

(22)  x(NaAlSi308)+4(CaAl2SiA)+KA18i3OH+2H2O= 

x(NaAlSisO8)+2(HCa2Al3SisO13)+H2KAl3Si3O12+2SiO2. 

It  is  impracticable  at  the  present  state  of  knowledge  to  write  reactions 
representing  the  changes  of  the  less  siliceous  plagioclases  into  more  sili- 
ceous plagioclases  and  other  minerals,  because  the  exact  compositions  of 
the  original  and  resultant  minerals  are  not  known.  The  alterations  of  the 

a  Clarke,  F.  W.,  The  constitution  of  the  silicates:  Bull.  U.  S.  Geol.  Survey  No.  125,  1895,  p.  29. 
»  Becke,  F.,  Ueber  Beziehungen  zwischen  Dynainometamorphose  und  Molecularvolumen:  Neues 
Jahrbuch  fur  Mineralogie,  etc.,  vol.  2,  1896,  p.  182. 


RELATIONS  OF  ALTERATIONS  OF  FELDSPARS.       265 

plagioclases  to  kaolinite,  equation  (1),  and  to  gibbsite,  equation  (2),  are 
by  reactions  of  carbonation  and  hydration;  the.  alterations  to  the  zeolites, 
equations  (4)  to  (14),  inclusive,  are  by  reactions  of  hydration,  and  (8) 
to  (14),  inclusive,  also  involve  carbonation.  The  alteration  to  prehnite, 
equation  (15),  is  a  reaction  of  hydration  and  desilication.  The  alterations 
to  zoisite  and  epidote,  equations  (16),  (17),  (18),  and  (19),  are  by  reactions 
of  hydration.  All  but  (1),  (3),  (16),  and  (17)  take  place  with  ncrease  of 
volume  ranging  from  1.58  to  52.76  per  cent,  provided  all  the  compounds 
formed  remain  in  situ.  All  take  place  with  liberation  of  heat.  The  altera- 
tions in  all  particulars  are  characteristic  of  the  zone  of  katamorphism. 

The  development  of  kaolin  is  probably  more  characteristic  of  the  belt 
of  weathering  than  of  the  belt  of  cementation.  The  development  of  the 
zeolites  and  cpidotes  is  known  to  occur  on  an  extensive  scale  in  the  belt  of 
cementation,  both  within  the  bodies  of  other  minerals  and  within  the  open- 
ings in  rocks.  Amygdules  and  veins  of  these  minerals,  with  quartz,  are  of 
great  importance  in  cementing  rocks.  The  material  for  this  work  is  doubt- 
less in  large  part,  though  not  altogether,  derived  from  feldspathic  minerals. 

For  the  intermediate  scapolites,  equations  (20)  and  (21),  the  alterations 
take  place  with  a  slight  increase  in  volume  for  marialite  and  a  slight 
decrease  for  meionite,  provided  all  the  compounds  which  enter  into  them 
are  solids.  The  reactions  are  those  of  silication  and  decarbonation  to  some 
extent,  and  this  involves  absorption  of  heat.  The  geological  occurrences 
correspond  with  the  physical-chemical  facts.  The  most  common  of  the 
scapolites  which  occurs  as  a  secondary  product  in  the  altered  rocks  is 
wernerite,  an  isomorphous  mixture  of  meionite  and  marialite  molecules. 
As  stated  by  Dana,  wernerite  "occurs  in  metamorphic  rocks,  and  most 
abundantly  in  granular  limestone  near  its  junction  with  the  associated 
granitic  or  allied  rock.""  Wernerite  is  associated  with  such  minerals  as 
pyroxene,  amphibole,  and  garnet,  which  occur  as  deep-seated  alterations. 
The  formation  of  wernerite  from  feldspar  is  probably,  therefore,  a  deep- 
seated  change  which  occurs  in  the  zone  of  anamorphism. 

The  alteration  of  orthoclase  and  plagioclase  together  to  albite,  zoisite, 
muscovite,  and  quartz,  equation  (22),  is  a  reaction  of  hydration  and  desili- 
cation. It  involves  an  increase  in  volume.  One  would  therefore  expect 
the  reaction  to  take  place  in  the  zone  of  katamorphism. 

"Dana,  J.  D.,  A  system  of  mineralogy;  Descriptive  mineralogy,  by  E.  S.  Dana;  Wiley  &  Sons, 
New  York,  6th  ed.,  1892,  p.  470. 


266  A  TREATISE  ON  METAMORPHISM. 

LEUCITE    GROUP. 

Leucite  is  the  only  rock-making  mineral  belonging  to  this  group. 

LEl'CITE. 

Leucite: 

KAlSijO,. 
Isometric. 
Sp.  gr.  2.45-2.5. 

occurrence. — Leucite  is  a  common  constituent  of  volcanic  rocks,  especially 
the  more  recent  ones.  The  fact  that  it  is  not  abundant  in  the  older  volcanic 
rocks  is  probably  due  to  its  ready  alteration.  It  may  have  been  present 
originally. 

Leucite  is  not  known  as  a  constituent  of  the  schists  and  gneisses 
derived  from  the  sediments.  Leucite  has  been  produced  by  pyro-chemical 
methods  from  analcite  and  potassium  chloride.  This  is  a  reversal  of  the 
reaction  in  the  case  of  change  of  leucite  to  analcite. 

Alterations. — Leucite  frequently  alters  to  analcite  (isometric;  sp.  gr.  2.22- 
2.29);  to  a  mixture  of  orthoclase  (monoclinic;  sp.  gr.  2.53-2.6)  and  kaolinite 
(monoclinic;  sp.  gr.  2.6-2.63);  to  a  mixture  of  orthoclase  and  muscovite 
(mouoclinic;  sp.  gr.  2.76-3.0);  and  to  a  mixture  of  orthoclase  and  nephe- 
lite  (hexagonal;  sp.  gr.  2.55-2.65). 

The  change  of  leucite  to  analcite  requires  a  substitution  of  sodium  for 
potassium;  hence  sodium  carbonate  or  some  other  sodium  compound  must 
be  supposed  to  be  present.  Supposing  sodium  carbonate  to  be  the  com- 
pound, the  reaction  is: 

(1)  2KAlSi2O6+NXCOs+2H,O=Na2Al2Si4O12.2H2O+K2COs+k. 

Ignoring  the  carbonates,  the  increase  in  volume  is  10.74  per  cent. 

The  passage  of  leucite  into  orthoclase  and  kaolin  is  as  follows,  sup- 
posing the  freed  potassium  to  unite  with  carbon  dioxide: 

(2)  4KAlSi2O6+CO2i-2H2O=2KAlSi3O8+H4Al2Si2O9+K2CO,+k. 

Supposing  the  potassium  carbonate  to  be  taken  into  solution,  the  decrease 
in  volume  of  the  orthoclase  as  compared  with  the  leucite  is  38.57  per  cent, 
and  the  decrease  of  the  orthoclase  and  kaolinite  together  as  compared  with 
the  leucite  is  10.58  per  cent. 

In  a  similar  way  the  passage  of  leucite  into  orthoclase  and  muscovite 
is  as  follows: 

(3)  6KAlSi3O,+CO2+H2O=3KAlSi,O8+KH2Al3Si3O12+K2COs+k. 


THE  ORTHOKHOMBIC  PYROXENES.  267 

As  before,  supposing  the  potassium  carbonate  to  be  taken  into  solution,  the 
decrease  in  volume  of  the  orthoclase  and  muscovite  as  compared  with  the 
leucite  is  12.43  per  cent. 

In  the  passage  of  leucite  into  orthoclase  and  nephelite  it  is  necessary 
to  suppose  that  a  part  of  the  potassium  of  the  leucite  is  replaced  by  sodium. 
Supposing  the  nephelite  formed  to  be  a  pure  soda-uephelite,  the  reaction 
would  be: 

(4)     4KAlSi2O6+Na2CO3=2KAlSisO8+2NaAlSiO4+K2CO3+k. 

Ignoring  the  carbonates,  the  decrease  in  volume  of  the  orthoclase  and 
nephelite  as  compared  with  the  leucite  is  7.59  per  cent. 

The  reactions  above  given,  except  the  last,  are  those  of  hydration, 
and  the  second  and  third  are  those  of  carbonation  also.  They  are, 
therefore,  reactions  which  are  to  be  expected  in  the  zone  of  katamorphism. 
The  change  of  leucite  into  orthoclase  and  nephelite  gives  decrease  in 
volume,  with  neither  hydration  nor  dehydration,  carbonation  nor  silication. 
It  is,  therefore,  to  be  expected  that  the  change  is  one  which  takes  place  in 
the  zone  of  anamorphism. 

The  artificial  transformation  of  leucite  into  analcite  by  treatment 
with  soda  solutions,  and  the  reverse  alteration  of  analcite  into  leucite 
by  treatment  with  potassium  solutions,  as  shown  by  Lemberg,0  is  an 
excellent  illustration  of  the  law  of  mass  action  and  proves  the  importance 
of  this  principle  under  natural  conditions. 

PYROXENE    GROUP. 

ORTHOKHOMBIC  PYROXENES. 

EN8TATITE,  BHON/ITK,  AXD  HYPERSTHENE. 

Enstatite: 

MgSiO2. 

Orthorhombic. 

Sp.  gr.  3.1-3.3. 
Bronzite: 

(MgFe)SiO3  where  Mg:Fe  :  :  8:1,  6:1,  and  3:1. 

Orthorhombic. 

Sp.  gr.  3.2-3.3. 
Hypersthene: 

(MgFe)SiO3  where  Mg:Fe  :  :  3:1,  nearly  to  1:1. 

Orthorhombic. 

Sp.  gr.  3.4-3.5. 

°  Lemberg,  J.,  Ueber  Silicatumwandlungen:  Zeitschr.  Deutsch.  geol.  Gesell.,  vol.  28,  1876,  pp. 
536-545. 


268  A  TREATISE  ON  METAMORPHISM. 

occurrence. — The  rhombic  pyroxenes  are  common  pyrogenic  constituents 
of  igneous  rocks  rich  in  magnesium.  They  are  common  in  the  normal 
diabases,  gabbros,  and  basalts,  and  are  abundant  in  the  norites,  peridotites, 
etc.  They  also  occur  in  the  intermediate,  basic,  and  ultrabasic  volcanic 
rocks,  including  both  lavas  and  tufts.  A  very  common  associate  of  the 
rhombic  pyroxenes  is  oliviue.  The  rhombic  pyroxenes  also  occur  in  the 
schists  and  gneisses,  especially  those  derived  from  eruptives.  In  such 
rocks  they  are  frequently  associated  with  the  monoclinic  pyroxenes.  They 
further  occur  as  vein  materials  and  are  found  in  meteorites. 

As  metamorphic  minerals,  enstatite  is  derived  from  pyrope  and 
hypersthene  from  almandite,  biotite,  and  common  garnet. 

Alterations. — The  most  frequent  alteration  of  the  rhombic  pyroxenes  is 
to  talc  (orthorhombic  or  monoclinic;  sp.  gr.  2.7-2.8).  The  less  frequent 
alterations  are  to  serpentine  (monoclinic;  sp.  gr.  2.50-2.65),  bastite  (ortho- 
rhombic;  sp.  gr.  2.50-2.75),  actinolite  (monoclinic;  sp.  gr.  3-3.2),  and 
anthophyllite  (orthorhombic;  sp.  gr.  3.1-3.2). 

For  the  sake  of  simplicity  it  is  assumed  that  where  pure  talc  or 
serpentine  is  produced  these  materials  are  derived  from  enstatite;  and  that 
where  bastite,  actinolite,  and  anthophyllite  are  produced  these  minerals  are 
derived  from  bronzite  or  hypersthene.  Of  course,  serpentine  or  talc  may 
be  produced  from  bronzite  or  hypersthene,  the  iron  separating  as  oxide  or 
carbonate.  One  such  possible  altei'ation  is  written.  However,  the  ordinary 
alterations  of  the  ferriferous  pyroxenes  are  to  bastite,  which  is  iron-bearing. 

The  change  of  enstatite  to  talc  is  as  follows : 

(1)  4MgSiOs+CO2+H20=H2Mg8Si4O,2+MgCOs-}-k. 

Supposing  the  magnesium  carbonate  to  be  dissolved,  the  increase  in 
volume  is  9.93  per  cent.  If  a  ferriferous  pyroxene  be  supposed  to  alter 
to  talc,  iron  oxide  must  separate.  Supposing  this  to  be  in  the  form  of 
magnetite  (isometric;  sp.  gr.  5.174),  and  supposing  that  the  magnesium 
is  to  the  iron  as  3:1,  or  that  the  mineral  is  intei'inediate  between  bronzite 
and  hypersthene,  the  reaction  may  be  written: 

(2)  3Mg3FeSi40I2+3H20+0=3H2Mg3Si40I2+Fes04-:-k. 

Similar  equations  may  be  written  by  which,  instead  of  magnetite, 
hematite  (rhornbohedral;  sp.  gr.  5.225)  or  limonite  (amorphous;  sp.  gr. 
3.80)  is  produced,  in  which  case  the  expansion  of  volume  would  be 


ALTERATIONS  OF  ORTHOKHOMBIC  PYROXENES.  269 

greater.  The  calculated  increase  in  volume  of  the  talc  and  magnetite,  as 
compared  with  the  pyroxene,  is  14.68  per  cent,  provided  the  average 
specific  gravity  of  bronzite  be  used,  and  21.73  per  cent  provided  the 
average  specific  gravity  of  hypersthene  be  used.  Probably  the  real 
increase  in  volume  is  the  average  of  the  above,  or  about  18.20  per  cent. 
In  the  case  of  a  hypersthene  in  which  the  iron  is  to  the  magnesium  as 
1:1  the  alteration  to  talc  may  be  as  follows,  provided  the  iron  separate  as 
magnetite :  , 

(3)  3MgFeSi2O6+H20+0=H2Mg,Si4O12+Fe3O4+2SiO,!+k. 

The  increase  of  volume  of  the  talc,  magnetite,  and  quartz  as  compared 
with  the  hypersthene  is  12.84  per  cent. 

Serpentine  is  produced  from  enstatite  by  the  following  reaction : 

(4)  3MgSiOs+2H,0=H4Mg8SiA+8iO,+k. 

In  case  the  Si02  is  dissolved,  the  increase  in  volume  is  14.25  per  cent ; 
if  it  separates  as  quartz  (rhombohedral ;  sp.  gr.  2.6535)  the  increase  in 
volume  is  38.36  per  cent. 

If  a  rhombic  pyroxene  be  taken  in  which  the  magnesium  is  to  the  iron 
as  3 : 1 — i.  e.,  stands  on  the  border  line  between  hypersthene  and  bronzite — 
serpentine  might  be  produced  by  the  following  reaction,  with  the  simul- 
taneous separation  of  hematite  and  quartz : 

(5)  2Mg3FeSiA2+4H2O+O=2H,MgsSiA+FeA+4SiO2+k. 

Using  the  specific  gravity  of  hypersthene,  in  case  only  serpentine  and 
hematite  separate  as  solids,  the  decrease  in  volume  is  2  21  per  cent,  and  if 
the  silica  separates  as  quartz  the  increase  of  volume  is  33.94  per  cent. 

Supposing  the  calcium  is  to  the  iron  as  1:1  and  the  excess  of  iron 
separates  as  magnetite,  the  reaction  is : 

(6)  3MgFeSi2O6+2H2O+O=H4MgsSiA+Fe3O4+4SiO2+k. 

The  increase  in  volume  of  the  serpentine,  magnetite,  and  quartz  as 
compared  with  the  hypersthene  is  20.24  per  cent. 

Other  reactions  may  be  written  which  represent  the  alterations  of 
bronzites  and  hypersthenes,  in  which  the  proportions  of  magnesium  and 
iron  are  different.  Also  reactions  may  be  written  in  which  the  oxide  of 
iron  forms  as  magnetite  or  limonite.  Where  magnetite  forms,  the  increase 
in  volume  would  be  less  than  for  hematite,  and  where  limonite  forms  the 
increase  in  volume  would  be  considerably  greater. 


270  A  TREATISE  ON  METAMORFHISM. 

If  in  the  formation  of  bastite,  a  pyroxene  be  taken  which  stands 
intermediate  between  bronzite  and  hypersthene — i.  e.,  in  which  the  magne- 
sium and  iron  are  as  3:1 — and  if  the  same  proportions  of  these  constituents 
be  supposed  to  hold  in  the  bastite,  the  reaction  is  as  follows: 

(7)  3Mg3FeSi<O12^8H2O=HI6Mg9Fe,Si8036+4SiO2^k. 

Using  the  specific  gravity  of  hypersthene,  if  the  silica  be  dissolved  the 
increase  of  volume  is  22  77  per  cent  (if  the  specific  gravity  of  bronzite  be 
employed,  15.65  per  cent);  if  the  silica  separates  as  quartz,  46.87  per  cent. 
Similar  reactions  may  be  written  which  represent  the  formation  of  bastites 
which  are  richer  and  poorer  in  iron,  in  which  cases  the  volume  changes  are 
slightly  different. 

The  passage  of  ferriferous  rhombic  pyroxene  into  anthophyllite  may 
be  one  of  pure  paramorphism,  since  in  anthophyllite  the  proportions  of 
magnesium  to  iron  have  ranges  paralleled  by  bronzite  and  hypersthene. 
Therefore,  the  only  necessary  change  is  a  molecular  one,  a  mineral  being 
produced  of  lower  symmetry  and  lower  specific  gravity  as  a  result  of  the 
alteration.  If  the  specific  gravity  of  hypersthene  be  used,  the  calculated 
increase  in  volume  due  to  the  lower  specific  gravity  of  the  resultant 
mineral  is  8.70  per  cent. 

In  the  formation  of  actinolite  from  a  rhombic  pyroxene,  it  is  necessary 
that  lime  and  silica  be  added.  Supposing  the  magnesium  is  to  the  iron  as 
3:1  in  both  the  rhombic  pyroxene  and  actinolite,  the  equation  is  as  follows: 

(8)  3Mg,FeSi4Oi2+4CaCO3+4Si02=Mg9Fe3Ca4Sii6O48+4CO,-f  k. 

The  decrease  in  volume  of  the  actinolite  as  compared  with  pyroxene, 
calcite,  and  quartz  is  7.40  per  cent  if  the  specific  gravity  of  hypersthene 
be  used,  and  if  that  of  bronzite  is  10.77  per  cent.  Similar  equations  may 
be  written  in  which  the  proportions  of  magnesium  and  iron  are  different. 

The  changes  of  the  rhombic  pyroxenes  to  talc  involve  reactions  of 
carbonation  and  hydration,  or  of  hydration  and  oxidation,  or  of  all  three 
together.  The  changes  of  the  rhombic  pyroxenes  to  serpentine  and  bastite 
involve  hydration  alone,  or  hydration  and  oxidation.  All  take  place  with 
increase  of  volume  and  liberation  of  heat, 

Corresponding  with  these  facts,  as  a  matter  of  observation  the  devel- 
opment of  serpentine,  bastite,  and  talc  from  the  rhombic  pyroxenes  takes 
place  in  the  zone  of  katamorphism.  The  development  of  talc  is  especially 
characteristic  of  the  belt  of  weathering,  and  serpentine  and  bastite  of  the 


MONOCLINIC  PYROXENES.  271 

belt  of  cementation,  although  it  can  not  be  asserted  that  the  formation  of 
any  of  these  minerals  is  confined  to  either  belt. 

The  paramorphic  change  of  rhombic  pyroxene  into  anthophyllite 
being  one  involving  lessening  of  specific  gravity  and  decrease  of  symmetry, 
one  would  expect  the  change  to  take  place  in  the  upper  physical-chemical 
zone,  but  I  have  been  unable  to  ascertain  from  the  literature  the  facts  in 
this  case. 

The  formation  of  actinolite  from  a  rhombic  pyroxene  requires  the 
assistance  of  calcite  and  silica.  This  reaction  is  one  of  silication  and 
decarbonation.  It  occurs  with  diminution  of  volume  and  absorption  of 
heat.  As  a  matter  of  observation,  corresponding  with  these  facts  it  is 
well  known  that  the  change  is  a  deep-seated  one. 

MONOCLINIC  PYROXEXEK. 
DIOPSIDE,  SAHLITE,  HEDEXBERUITK,  Al'UITE,  ACMITE,  SPODUMEJfE,  WOLLASTOXITE,  AXD  PECTOLITE. 

Diopside: 

CaMgSi2O6- 
Monoclinic. 
Sp.  gr.  3.2-3.38. 
Sahlite.: 

Ca(MgFe)Si206. 
Monoclinic. 
Sp.  gr.  3.25-3.4. 
Hedenberyite: 

CaFeSi2O6- 

Monoclinic. 

Sp.  gr.  3.5-3.58. 
Augite: 

Ca(MgFe)Si2O6with  (MgFe)  (AlFe)2SiO6. 

Monoclinic. 

Sp.  gr.  3.3-3.5. 
Acmite: 

NaFeSi206. 

Monoclinic. 

Sp.  gr.  3.50-3.55. 
Spodumene: 

LiAlSi2Og- 

Monoclinic. 

Sp.  gr.  3.13-3.20. 
Wollaslonite: 

CaSiOs. 

Monoclinic. 

Sp.  gr.  2.8-2.9. 
Pectolite: 


Monoclinic. 
Sp.  gr.  2.68-2.78. 


272  A  TREATISE  ON  METAMORPBISM. 

The  minerals  diopside,  sahlite,  and  augite  constitute  the  so-called 
diopside-augite  series. 

occurrence. — The  pyroxene  group  is  one  of  the  most  widespread  and 
important.  One  or  another  variety  of  pyroxene  may  occur  in  almost  any 
rock;  but  the  pyroxenes  are  much  more  abundant  in  the  intermediate  and 
basic  than  in  the  acidic  rocks.  Pyroxene  is  found  in  the  plutonic  and 
volcanic  rocks,  as  an  original  constituent  of  the  clastic  rocks,  and  as  an 
original  and  secondary  constituent  of  the  metamorphosed  rocks,  both  of 
igneous  and  of  aqueous  origin.  The  minerals  of  the  pyroxene  group  occur 
extensively  in  veins. 

Diopside  occurs  in  marbles,  especially  magnesian  marbles.  Indeed, 
this  is  the  common  form  of  pyroxene  which  develops  as  a  secondary 
constituent  during  the  metamorphism  of  the  magnesian  limestones.  It  also 
occurs  in  veins.  As  a  metamorphic  mineral  diopside  is  derived  from 
dolomite. 

Sahlite  occurs  in  ferriferous  magnesian  marbles.  Like  diopside,  it  is 
also  found  in  veins.  Unlike  diopside,  it  is  a  common  product  in  many 
horublendic  schists  and  gneisses,  such  rocks  probably  having  been  in  their 
original  condition  calcareous,  magnesian,  and  ferriferous.  Sahlite  is  derived 
from  ankerite  and  parankerite. 

Hedenbergite  occurs  as  a  rather  common  constituent  of  some  nepheline 
syenites  and  other  basic  syenites. 

Augite  is  a  common  form  of  pyroxene  in  the  eruptive  rocks,  both 
plutonic  and  volcanic.  It  occurs  in  many  mechanical  sediments.  It  also 
is  found  in  metamorphic  rocks  of  both  igneous  and  sedimentary  origin, 
though  in  the  sedimentary  metamorphosed  rocks  it  is  less  common  than 
diopside  and  sahlite.  But  augite  develops  to  a  considerable  extent  in  the 
sedimentary  rocks  which  are  intermediate  between  the  chemical  and 
mechanical  rocks — that  is,  those  which  contain  abundant  calcium  carbonate 
and  also  are  rich  in  aluminum.  Augite  is  recorded  as  a  metamorphic 
mineral  derived  from  hornblende. 

Wollastonite  occurs  especially  in  the  metamorphosed  calcareous  and 
sedimentary  rocks,  it  being  a  secondary  product  produced  by  metamorphism. 
It  is  found  abundantly  in  marbles,  and  in  schists  and  gneisses  which  were 
originally  calcareous,  especially  the  calcareous  feldspathic  schists.  It  also 
develops  in  calcareous  inclusions  in  eruptive  rocks,  and  is  found  as  a  contact 


ALTERATIONS  OF  DIOPSIDE-AUGITE  SERIES.  273 

product  of  igneous  and  calcareous  rocks.     The  schists  and  gneisses  contain- 
ing wollastonite  are  often  garnetiferous  and  epidotic. 

The  very  frequent  development  of  the  above  pyroxenes  in  the  sedi- 
mentary rocks  which  are  calcareous,  rather  than  amphiboles,  is  due  to  the 
fact  that  the  pyroxenes  are  richer  in  calcium  than  are  the  amphiboles.  Where 
sedimentary  rocks  contain  magnesium  abundantly  with  the  calcium,  the 
amphiboles  are  likely  to  form  rather  than  the  pyroxenes. 

Acmite  occurs  mainly  in  the  eruptive  rocks,  and  especially  in  those 
which  are  rich  in  alkalies.  According  to  Rosenbusch,  it  occurs  especially 
in  granites  and  syenites  rich  in  sodium,  in  the  ela3olite- syenites,  phonolites, 
and  leucitophyres.  As  a  metamorphic  mineral  acmite  is  derived  from 
arfvedsonite. 

Spodumene  sometimes  occurs  as  an  accessory  constituent  in  the  granites, 
schists,  and  gneisses,  and  in  some  cases  as  considerable  masses. 

Pectolite,  while  not  an  abundant  mineral,  is  present  as  a  secondary 
constituent  in  many  basic  eruptive  rocks,  both  plutonic  and  volcanic.  It  is, 
however,  especially  prevalent  in  the  volcanic  rocks,  since  these  are  more 
porous,  and  pectolite  is  especially  likely  to  occur  in  cavities  or  seams. 
Occasionally  pectolite  is  found  in  the  metamorphic  rocks  as  a  product  of 
apophyllite. 

Alterations  of  the  diopside-augite  series. — The  most  common  alteration  of  the  non- 
aluminous  diopside  and  sahlite  is  into  talc  (orthorhombic  or  monoclinic; 
sp.  gr.  2.7-2.8).  They  also  often  alter  into  serpentine  (monoclinic;  sp.  gr. 
2.5-2.65).  These  changes  are  accompanied  by  the  formation  of  calcium 
carbonate,  and  frequently  by  the  separation  of  a  part  of  this  carbonate  as 
calcite  (rhombohedral ;  sp.  gr.  2.7135). 

The  aluminous  pyroxenes,  augite,  and  diallage,  under  the  conditions  of 
the  zone  of  katamorphism,  change  into  chlorite  (monoclinic;  sp.  gr.  2.08- 
2.16'),  with  which  are  usually  associated  epidote  (monoclinic;  sp.  gr.  3.25- 
3.5),  this  mineral  often  being  embedded  in  the  chlorite  and  calcite.  Under 
conditions  of  weathering,  any  of  the  minerals  of  the  diopside-augite  series 
may  be  partly  or  entirely  replaced  by  quartz  (rhombohedral;  sp.  gr. 
2.6535),  chalcedony  (cryptocrystalline;  sp.  gr.  2.6-2.64),  or  calcite.  Such 
replacements  are  particularly  common  in  the  case  of  the  porous  andesites 
and  trachytes,  and  also  in  tuffs.  Not  infrequently  this  replacement  of 
the  pyroxene  occurs  without  the  feldspar  being  greatly  affected. 
MON  XLVII — 04 18 


274  A  TREATISE  ON  METAMORPHISM. 

However,  perhaps  the  most  frequent  and  characteristic  of  the  altera- 
tions of  the  diopside-augite  series  is  uralitization  or  change  to  amphibole 
(monoclinic;  sp.  gr.  2.9-3.4).  This  process  is  particularly  characteristic  of 
the  ancient  igneous  rocks,  and  especially  those  which  are  under  compara- 
tively deep-seated  conditions,  although  the  alteration  is  by  no  means  con 
fined  to  deep-seated  rocks.  It  occurs  on  a  great  scale  under  the  conditions 
of  the  transformation  of  the  igneous  rocks  into  schists  and  gneisses.  During 
the  process  of  uralitization  epidote  also  very  frequently  forms.  Not  infre- 
quently also  magnetite  (isometric;  sp.  gr.  5.16—5.18)  and  calcite  separate. 
In  some  cases  the  change  is  accompanied  by  the  development  of  a  feldspar, 
such  as  albite  (triclinic;  sp.  gr.  2.62-2.65).  The  kind  of  amphibole  which 
forms  depends  upon  the  variety  of  the  pyroxene.  From  diopside,  tremolite 
(monoclinic;  sp.  gr.  29-3.1)  is  the  ordinary  product;  from  sahlite,  actino- 
lite  (monoclinic;  sp.  gr.  3.0-3.2)  is  normally  to  be  expected;  from  diallage 
and  omphacite  (according  to  Zirkel,"  varieties  of  augite),  smaragdite  (a 
variety  of  hornblende)  is  ordinarily  produced;  and  from  ordinary  augite, 
hornblende  (monoclinic;  sp.  gr.  3.05—3.47)  is  usually  developed.  Finally, 
it  not  infrequently  occurs  that  augite  changes  directly  into  biotite  (mono- 
clinic;  sp.  gr.  2.90). 

The  change  of  diopside  to  talc  may  be  written  as  follows : 

(1 )  3CaMgSi2O64-3CO2  f  H2O=H2MgsSi4Oi2+3CaCOs+2SiO2+k. 

The  increase  in  volume,  supposing  all  compounds  remain  as  solids,  is  48.74 
per  cent.  If  only  the  talc  remains,  the  decrease  is  30.13  per  cent.  Sup- 
posing the  diopside  were  one  in  which  a  part  of  the  calcium  and  magnesium 
were  replaceable  by  iron,  so  that  the  calcium  and  magnesium  and  iron  are 
present  in  equal  proportion,  thus  approaching  sahlite  in  composition,  and 
supposing  the  iron  to  pass  into  magnetite,  the  reaction  is— 

(2)  3CaMgFeSi,O9+3C02+H2O+O=H2Mg,SiA2+3CaCO,+Fe3O4+5SiO2+k. 

The  increase  in  volume  .of  the  talc,  calcite,  magnetite,  and  quartz,  as  com- 
pared with  the  diopside,  is  27.88  per  cent. 

The   change  of  diopside    to  serpentine  may  be  represented  by  the 
following  equation: 

(3)  3CaMgSiA+3C02+2HaO=H4Mi?3SiA+4'Si02+3CaC03+k. 
"Xaumann,  C.  F.,  and  Zirkel,  F.,  Elementeder  Mineralogie,  Leipzig,  1898,  p.  696. 


ALTERATIONS  OF  DIOPSIDE-AUGITE  SERIES.  275 

Supposing  all  the  compounds  separated  as  solids,  the  increase  in  volume  is 
f>6.32  per  cent.  If  only  the  serpentine  and  quartz  remain  as  solids,  the 
increase  in  volume  is  U.44  per  cent. 

The  change  from  sahlite  to  ferriferous  bastite,  provided  that  in  both 
compounds  the  magnesium  is  to  the  iron  as  3  :  1,  is — 

(4)  3Ca4Mg3FeSiA4+12CO2+8H2O  =  H16Mg9FesSi8OS6+16SiO2+12CaCO3+k. 

If  all  the  compounds  separate  as  solids,  the  increase  in  volume  is  56.41  per 
cent;  if  the  bastite  and  quartz  remain  as  solids,  1.93  per  cent.  Supposing 
the  calcium,  magnesium,  and  iron  were  in  equal  proportions  in  the  sahlite, 
and  that  in  the  bastite  the  magnesium  were  to  the  iron  as  3  :  1,  the  equation 
may  be  written — 

(5)  9CaMgFeSi3O9+9CO.!+8H.1O+2O=H16Mg9FesSi8O36+2FesO4+9CaCO3+19SiO.,+k. 

The  increase  in  volume  of  the  serpentine,  magnetite,  calcite,  and  quartz, 
as  compared  with  the  sahlite,  is  37.50  per  cent. 

It  is,  of  course,  not  impossible  that  serpentine  shall  develop  as  one  of 
the  products  from  augite.  In  this  case  it  doubtless  forms  from  the  sahlite 
molecule  of  the  augite  compound,  the  sesquioxide  compounds  passing  into 
some  other  mineral.  It  hardly  seems  advisable  to  attempt  to  write  equa- 
tions representing  such  an  alteration. 

In  writing  equations  for  the  alterations  of  the  aluminous  pyroxenes  into 
chlorite  and  epidote  it  is  necessary  that  certain  assumptions  shall  be  made 
in  reference  to  the  relative  proportions  of  the  various  elements.  Moreover, 
if  equations  are  written  which  produce  chlorite  alone,  a  large  amount  of  the 
sesquioxide  bases  must  be  left  over.  If  an  equation  be  written  for  the  forma- 
tion of  epidote,  a  large  amount  of  magnesium  is  unaccounted  for.  Since  it 
is  very  common  for  the  minerals  chlorite  and  epidote  to  form  simultaneously, 
an  equation  is  written  on  this  supposition.  In  order  to  give  definiteness  to 
the  compound,  it  is  supposed  that  there  are  twice  as  many  molecules  of  the 
diopside  part  of  the  augite  molecule  as  of  the  other  part.  Furthermore,  it 
is  supposed  for  the  diopside  molecule  that  the  magnesium  is  to  the  iron  as 
2:1;  and  for  the  other  molecule  that  the  aluminum  is  to  the  iron  sesquioxide 
as  3:1.  An  epidote  is  taken  in  which  the  aluminum  is  to  the  iron  as  2:1. 
A  chlorite  between  clinochlore  and  prochlorite  is  taken,  as  such  a  chlorite 
is  at  about  the  middle  of  the  series.  As  may  be  seen  by  reference  to  the 
analyses  of  augites  and  epidotes,  the  proportions  taken  represent  about  their 


276  A  TREATISE  ON  METAMORPHISM. 

average  compositions.  With  all  these  hypotheses,  and  supposing  the  extra 
silica  to  separate  as  quartz,  the  magnesia  to  separate  as  magnesium  carbonate, 
and  the  iron  as  sesquioxide  of  iron,  the  equation  may  be  written  as  follows: 

(6)     6[2(Ca,Mg2FeSi6018).Mg4Fe2Al9FesSi60S6]-H2C02+39H20+120= 


If  all  the  compounds  remain  as  solids  the  increase  in  volume  is  15.43  per 
cent.  If  the  magnesium  carbonate  be  dissolved  the  increase  in  volume  is 
8.58  per  cent. 

It  is  evident  that  many  other  equations  could  be  written  if  other  sup- 
positions be  made  as  to  the  relative  proportions  of  the  magnesium  to  the 
iron  and  the  aluminum  to  the  iron  in  the  respective  compounds,  and  if  other 
chlorite?  than  the  particular  one  chosen  be  produced.  For  the  complex 
silicates,  present  knowledge  is  not  sufficient  to  determine  whether  or  not 
particular  equations  written  accurately  represent  the  alterations  which  take 
place,  although  closer  study  in  the  future  may  possibly  determine  this. 
But  there  is  little  doubt  that  substantially  the  change  represented  by  equa- 
tion (6)  has  occurred  in  many  instances,  whether  it  can  be  verified  in  an 
individual  case  or  not,  as  doubtless  have  also  a  multitude  of  alterations 
which  might  be  represented  by  other  possible  equations.  The  difficulty  is 
to  ascertain  in  a  given  instance  which  of  the  equations  represents  a  given 
alteration.  It  is  hoped  that  the  quantitative  statement  of  the  problem  given 
by  equation  (6)  and  following  equations  will  lead  to  closer  study  of  the 
compounds  which  enter  into  new  compounds  and  the  compounds  which  are 
produced,  and  thus  to  more  exact  knowledge  of  the  various  alterations  of 
augite. 

According  to  the  above  reactions,  as  would  be  expected  from  the  nature 
of  the  compounds,  the  alteration  of  diopside  and  sahlite  more  frequently 
produces  talc,  serpentine,  and  bastite,  while  the  alteration  of  augite  more 
frequently  produces  chlorite  and  epidote. 

As  already  noted,  perhaps  the  most  characteristic  of  the  alterations  of 
the  pyroxenes  is  to  the  amphiboles.  This  alteration  involves  the  substitu- 
tion of  magnesium,  or  magnesium  and  iron,  for  calcium.  It  is  supposed 
that  the  iron  and  magnesium  are  added  in  the  form  of  carbonate,  and  that 
the  liberated  calcium  separates  in  the  form  of  carbonate.  Parallel  equa- 
tions can,  however,  readily  be  written  on  the  basis  of  any  other  magnesium 
compound  being  added  and  similar  iron  and  calcium  compounds  being 


ALTERATIONS  OF  DIOPSIDE-  AUGITE  SERIES.  277 

produced.  It  is  assumed,  further,  that  the  alteration  of  a  pyroxene  results 
in  the  production  of  the  most  closely  allied  amphibole.  Of  course  this  is 
not  always  the  fact,  but  it  is  believed  to  be  usual.  Following  this  assump- 
tion, the  alteration  of  diopside  is  to  tremolite,  of  sahlite  is  to  actinolite,  of 
augite  is  to  hornblende. 

The  change  from  diopside  to  tremolite  may  be  written  as  follows: 

(7)  2CaMgSiA+MgCO,=CaMg3Si4O,2+CaCO3+k. 

Regarding  the  magnesium  carbonates  as  added  in  solution  and  the  calcium 
carbonate  as  subtracted  in  solution,  the  increase  in  volume  is  5.68  per  cent. 
If  the  magnesium  carbonate  be  considered  as  present  as  magnesite,  and  the 
calcium  carbonate  be  considered  as  present  as  calcite,  the  increase  in  volume 
is  10.55  per  cent. 

The  change  from  sahlite  to  actinolite,  supposing  the  magnesium  and 
iron  to  be  present  in  the  sahlite  in  equal  proportions,  is  as  follows  : 

(8)  2Ca2MgFeSi4012+FeCO3-t-  MgC03=Ca2Mg3Fe3Si8021+2CaCO3+k. 

Supposing  the  sahlite  and  actinolite  only  to  be  solids,  the  increase  in  vol- 
ume is  7.28  per  cent.  If  all  the  compounds  are  regarded  as  solids  on  both 
sides  of  the  equation,  the  increase  in  volume  is  10.81  per  cent. 

Supposing  that  in  the  augite  compound  there  are  two  of  the  sahlite 
molecules  to  one  of  the  sesquioxide  molecule,  and  supposing  that  the  mag- 
nesium and  iron  are  in  equal  proportions  in  both  the  augite  and  hornblende, 
the  general  alteration  may  be  written  as  follows: 

(9)  2[Ca!!MgFeSi4012.  (MgFe)  (AlFe)2  Si06]+FeCO3rMgCO3= 

.-  (MgFe)2  (AlFe)4Si2O12+2CaCO3+k. 


But  before  the  volume  relations  can  be  calculated  it  is  necessary  to 
assume  definite  proportions  between  Mg  and  Fe,  and  Al  and  Fe,  in  the 
second  members  of  the  augite  and  hornblende  molecules.  If  the  magne- 
sium be  taken  to  the  iron  as  2:1  and  the  aluminum  to  the  iron  as  2:1, 
an  average  case,  and  only  the  augite  and  hornblende  be  considered  as 
solids,  the  increase  in  volume  is  4.30  per  cent.  If  all  the  compounds  in 
both  equations  are  solids,  the  increase  in  volume  is  6.14  per  cent. 

An  inspection  of  the  above  equations  giving  the  alterations  of  the 
diopside-augite  series  to  amphibole  shows  that  the  chemical  change  in  the 
alteration  of  diopside  and  sahlite  to  tremolite  and  actinolite  is  relatively 
greater  than  in  the  alteration  of  augite  to  hornblende.  Moreover,  if  it  be 


278  A  TREATISE  ON  METAMOKPHISM. 

supposed  that  the  last  half  of  the  augite  and  hornblende  molecules  are 
present  in  greater  proportion  than  given  in  the  equations,  the  chemical 
change  would  be  of  still  less  relative  importance.  This  is  of  interest  because 
the  alteration  of  augite  to  hornblende  is  a  far  more  common  phenomenon 
than  the  alterations  of  diopside  and  sahlite  to  tremolite  and  actinolite  The 
equations  also  give  reasons  for  the  very  frequent  occurrence  of  calcite  with 
uralite.  The  nature  of  the  alterations  is  such  that  calcium  carbonate  must 
be  produced,  and  very  naturally  a  portion  of  this  substance  frequently 
separates  as  calcite. 

In  the  change  of  augite  to  biotite  it  is  necessary  that  potassium  be 
derived  from  some  source.  Supposing  it  to  be  furnished  in  the  form  of 
potassium  carbonate,  as  a  result  of  the  decomposition  of  some  of  the  potas- 
sium-bearing silicates,  the  simplest  form  of  reaction  may  be  written  as 
follows  : 


(10)     2[Ca(MgFe)Si2O6.(MgFe)(AlFe)2Si06] 

2HK(MgFe),(AlFe),Si,O,a+2CaCO,+k. 

Supposing  the  MgO:  FeO:  :  2:  1,  and  the  A12O3:  Fe2O3:  :  3:  1  —  these  ratios 
being  chosen  because  they  represent  about  an  average  of  the  analyses  — 
and  multiplying  the  above  equation  by  6,  we  have: 

(  11  )     2[CaflMg4Fe2Si,20S6.Mg4Fe2Ali(FesSi60S6]  +6K2CO,+6H2O+6CO!= 
2(  H6K6Mg8Fe4Al9FesSil8OT2  )  -f  12CaCOs+k. 

Disregarding  all  other  compounds,  the  increase  in  volume  of  the  biotite  as 
compared  with  the  augite  is  17.26  per  cent. 

The  alteration  of  diopside  and  sahlite  to  talc,  serpentine,  and  bastite, 
equations  (1),  (3),  and  (4),  all  involve  increase  in  volume  and  liberation  of 
heat;  also  they  are  alterations  involving  carbonation  and  hydration.  Equa- 
tions (2)  and  (5)  involve  carbonation,  hydration,  and  oxidation.  In  all 
except  equation  (1),  even  if  all  of  the  separated  quartz  and  calcite  is 
dissolved,  there  is  still  an  increase  in  volume.  They  therefore  stand  as 
alterations  that  are  typical  of  all  the  principles  of  metamorphism  in  the 
zone  of  katamorphism. 

The  changes  of  the  pyroxenes,  especially  augite,  to  chlorite  and  epidote, 
equation  (6),  involve  hydration,  carbonation,  and  oxidation.  The  change 
occurs  with  increase  in  volume  and  liberation  of  heat,  even  if  the  resultant 
oxide  of  iron  and  magnesium  carbonate  be  ignored.  If  these  separate  as 
solids,  the  increase  in  volume  is  considerable.  The  alteration  is,  therefore, 


ALTERATIONS  OF  DIOPSIDE-AUGITE  SERIES.  279 

like  the  change  to  talc,  serpentine,  and  bastite,  one  characteristic  of  the 
upper  physical-chemical  zone.  The  change  of  pyroxene  to  the  fibrous 
aniphibole  known  as  uralite  occurs  in  the  belt  of  cementation  on  an  exten- 
sive scale,  and  to  this  position  the  volume  change  corresponds. 

But  the  passage  of  pyroxene  into  definite  amphibole  individuals  is  one 
of  the  most  common  alterations  in  the  zone  of  anamorphism,  and  especially 
under  conditions  of  mashing.  The  rule  for  this  zone  is  for  alterations  to 
occur  which  result  in  minerals  of  higher  specific  gravity.  The  alteration 
of  pyroxene  to  amphibole  seems  to  be  an  exception  to  this  rule;  for  the 
specific  gravity  of  the  pyroxenes  ranges  between  3.2  and  3.6,  while  that  of 
the  amphiboles  varies  from  2.9  to  3.4. 

Mainly  in  consequence  of  this  decrease  in  specific  gravity  the  increase 
in  volume,  as  already  seen,  of  all  compounds  entering  into  the  reactions  in 
the  change  from  diopside  to  tremolite,  equation  (7),  is  10.55  per  cent;  of 
sahlite  to  actinolite,  equation  (8),  10.81  per  cent;  of  augite  to  hornblende, 
equation  (9),  6.14  per  cent,  supposing  that  the  necessary  chemical  constitu- 
ents added  to  the  pyroxene  are  solid  carbonates  and  the  other  compounds 
produced  are  solid  carbonates. 

Unlike  the  previous  alterations,  these  changes  do  not  involve  oxidation, 
hydration,  or  carbonation ;  nor,  on  the  other  hand,  do  they  involve  deoxi- 
dation,  dehydration,  or  silication  They  are  substitution  reactions,  by  which 
magnesium,  or  iron,  or  both  take  the  place  of  calcium.  They  are,  there- 
fore, analogous  to  the  dolomitization  or  ferritization  of  the  limestones;  but 
the  volume  change  is  in  an  opposite  sense  from  those  alterations. 

But  another  factor  may  enter  into  the  problem,  the  effect  of  which  is 
hard  to  estimate.  The  exchange  of  the  magnesium  and  iron  for  calcium  is 
supposed  to  take  place  with  the  separation  of  a  carbonate.  If  such  carbon- 
ate were  simultaneously  silicated,  the  entire  volume  change  for  all  the  fac- 
tors concerned  would  be  decrease.  It  is  necessary  to  consider  the  volume 
relations  of  all  the  resultant  minerals  rather  than  those  of  the  pyroxene  and 
amphibole  alone,  and  hence  it  may  be  that  in  the  change  of  pyroxene  to 
amphibole  in  the  lower  physical-chemical  zone,  if  one  could  ascertain  the 
entire  effect  of  this  alteration  in  connection  with  other  alterations,  the 
volume  would  not  be  expanded  but  contracted,  and  thus  there  be  no  real 
exception  to  the  law  that  the  reactions  here  take  place  with  condensation 
of  volume. 


280  A  TREATISE  ON  METAMORPHISM. 

But,  even  if  this  be  true,  it  is  freely  admitted  that  the  case  is  not  fully 
covered,  for  it  is  very  uncommon  indeed  for  the  chief  resultant  mineral  of 
an  alteration  in  the  zone  of  anamorphism  to  have  a  lower  specific  gravity 
than  the  minerals  from  which  it  is  derived  with  comparatively  small  chem- 
ical change.  Apparently,  for  some  reason  the  amphiboles  are  more  stable 
under  conditions  of  moderately  deep-seated  metamorphism  than  the  pyrox- 
enes. This  view  is  confirmed  by  the  fact  that,  while  the  majority  of  the 
schists  and  gneisses  are  amphibolitic  rather  than  pyroxeuitic,  in  some  of  the 
gneisses  and  schists  which  have  been  altered  under  very  deep-seated  condi- 
tions the  pyroxenes  are  present  instead  of  the  amphiboles.  The  significance 
of  this  fact  is  probably  that  an  unusually  high  pressure  is  required  in  order 
to  produce  the  mineral  of  the  highest  specific  gravity  in  the  case  of  the 
pyroxene-amphibole  group. 

The  change  from  augite  to  biotite,  equations  (10)  and  (11),  is  one  which 
takes  place  in  the  zone  of  anamorphism  especially  under  conditions  of 
mashing.  In  this  change  the  volume  of  the  biotite  produced  is  greater 
than  that  of  the  pyroxene;  in  the  case  of  the  equation  (11)  17.26  per  cent. 
However,  this  case  is  similar  to  that  of  hornblende.  Potassium  salt  must 
be  added  from  some  other  mineral  and  a  calcium  salt  is  produced.  In 
order  to  get  the  real  volume  relation  of  the  reaction  it  would  be  necessary 
to  know  the  source  of  the  potassium  and  the  place  to  which  the  calcium 
goes;  and  as  present  information  does  not  enable  us  to  determine  this,  no 
definite  statement  can  be  made  as  to  the  total  effect  of  all  the  changes 
involved  in  the  alteration  of  augite  to  biotite. 

Alterations  of  pyroxenes  other  than  the  diopside-augite  series. No    6qUatioilS     ai'6     Written 

for  the  alterations  of  wollastonite,  hedenbergite,  acmite,  and  pectolite, 
because  the  character  of  the  alterations  of  these  compounds  has  not  been 
described  in  the  standard  authorities,  although  there  is  no  doubt  that  these 
minerals,  like  all  others,  do  undergo  various  alterations.  All  these  minerals 
form  under  deep-seated  conditions;  and  it  is  to  be  expected  that  under  the 
conditions  of  the  zone  of  katamorphism,  especially  in  the  belt  of  weathering, 
they  would  be  decomposed;  but,  if  so,  the  minerals  into  which  they  change 
are  unknown. 

Alterations  of  spodumeue  are  recorded.  According  to  Dana,  the  first 
stage  in  the  alteration  of  spodumene  is  to  beta-spodumene  (crystallization 
not  determined;  sp.  gr.  2.644-2.649),  in  which  one-half  of  the  lithium  is 


ORTHORHOMBIC  AMPHIBOLES.  281 

replaced  by  sodium.  The  second  stage  in  the  process  of  alteration  is  the 
beta-spodumene  passing  into  eucryptite  (hexagonal;  sp.  gr.  2.667)  and  albite 
(triclinic;  sp.  gr.  2.62-2.65),  or  into  muscovite  (monoclinic;  sp.  gr.  2.76-3) 
and  albite,  the  uniform  mixture  of  which  has  been  known  as  cymatolite 
(sp.  gr.  2.69—2.70);  or  spodumeiie  may  pass  into  muscovite  and  microcline 
(triclinic;  sp.  gr.  2.54-2.57).  The  reactions  representing  the  above  changes 
may  be  expressed  in  the  manner  shown  by  the  equations  given  below. 
The  change  from  spodumene  to  beta-spodumene  may  be  written: 

(12)  4IJAlSi2O6+N%CO3=2LiNaAl2SiiO12+Li2CO,,+k. 

The  increase  in  volume  is  24.72  per  cent.     Where  the   beta-spodumene 
breaks  up  into  eucryptite  and  albite  the  reaction  is: 

(13)  LiXaAl2Si4O,2=LiA1SiO,+NaAlSi3O8+k. 

The   increase  in  volume  is  0.05   per  cent.     Where  the  beta-spodumene 
passes  into  muscovite  and  albite  the  reaction  is: 

(14)  6LiNaAl2Si4O,2+K2CO3+2H.,O+2CO2=2H2KAl3Si3OI2+6NaAlSi3O8+3Li2CO3+k. 

The  decrease  in  volume  is  0.76  per  cent.     In  case  the  spodumene  changes 
into  muscovite  and  microcline  the  reaction  is: 

(15)  12LiAlSi2O6+4K2COi,+2C02+2H2O=2H!!KAl3Si3O1;i+6KA]Si308+6Li2COs+k. 

The  increase  in  volume  is  31.74  per  cent. 

One  would  expect  reactions  (12)  and  (15)  to  take  place  in  the  zone  of 
katamorphism,  but  I  know  of  no  observations  on  this  point,  nor  as  to  the 
conditions  under  which  reactions  (13)  and  (14)  occur. 

AMPHIBOLE   GROUP. 
ORTHORHOMBIC  AMPHIBOLES.  . 
ANTHOPHYLLITE  AND  UEDBITE. 

Anthophyllite: 

(MgFe)  SiO3  Mg :  Fe : :  4: 1,  3  : 1,  etc. 

Orthorhombic. 

Sp.  gr.  3.1-3.2. 
Oedrite: 

( MgFe )  jSij06.  MgAl2SiO6. 

Orthorhombic. 

Sp.  gr.  3.1-3.2. 

occurrence. — Aiithopliyllite  and  gedrite  occur  in  the  schists  and  gneisses, 
both  those  derived  from  sedimentary  and  those  derived  from  igneous  rocks. 


282  A  TREATISE  ON  METAMORPHISM. 

They  are  frequently  associated  with  hornblende  and  mica.  Anthophyllite 
occupies  the  same  position  in  the  rhombic  amphiboles  that  bronzite  does  in 
the  rhombic  pyroxenes,  and  gedrite  the  same  position  as  hypersthene.  As 
already  described  (p.  270),  the  bronzites  and  hypersthenes  alter  into 
anthophyllite.  It  is  to  be  expected  that  gedrite  in  a  similar  manner  forms 
from  hypersthene,  but  this  particular  alteration  is  not  mentioned  in  the 
standard  books  of  reference.  Also,  as  described  (p.  310),  anthophyllite 
forms  as  a  secondary  product  from  olivine. 

Alterations. — Anthophyllite  by  hydration  passes  into  talc  (orthorhombic 
or  monoclinic;  sp.  gr.  2.75)  or  bastite  (orthorhombic;  sp.  gr.  2.6).  Also, 
Lacroix  states"  that  rarely  it  alters  into  calcite  (rhombohedral ;  sp.  gr. 
2.7135).  Supposing  the  magnesium  is  to  the  iron  as  3:1,  and  that  the 
freed  iron  separates  as  hematite  (rhombohedral;  sp.  gr.  5.225),  the  alteration 
to  talc  may  be  written  as  follows: 

(1)  2Mg9FeSi4O12+2H2O+O=2H2MgsSi)O12+Fe2O3  f  k. 

The  increase  in  volume  of  the  talc  and  hematite,  as  compared  with  the 
anthophyllite,  is  11.41  per  cent.  If  the  iron  oxide  be  supposed  to  be 
hydrated  into  limonite  (not  crystallized;  sp.  gr.  3.80),  the  increase  in 
volume  would  be  still  greater.  If  bastite  be  produced,  and  it  be  supposed 
that  the  magnesium  is  to  the  iron  as  3:1,  the  same  as  in  the  anthophyllite, 
the  equation  may  be  written: 

(2)  3MgsFeSi4O1.2+8H2O=H16Mg9FesSi8OS6+4Si02+k. 

The  increase  in  volume  of  the  bastite  and  quartz  (rhombohedral;  sp.  gr. 
2.6535)  as  compared  with  the  anthophyllite  is  34.09  per  cent.  If  the  silica 
be  supposed  to  be  dissolved  the  increase  in  volume  is  12.09  per  cent. 

The  particular  alterations  which  gedrite  undergoes  are  not  described 
in  the  standard  text-books;  therefore  no  attempt  is  made  to  write  equations 
for  changes  of  this  mineral. 

The  alteration  of  anthophyllite  to  talc  and  iron  oxide  involves  hydra- 
tion and  oxidation.  The  alteration  of  anthophyllite  to  bastite  involves 
hydration  and  desilication.  Both  sets  of  reactions  are,  therefore,  char- 
acteristic of  the  zone  of  katamorphism;  and  it  is  in  this  zone,  especially  in 
the  belt  of  weathering,  that  the  changes  occur. 

"  Lacroix,  A.,  Min£ralogie  de  la  France,  Paris,  1893-95,  vol.  1,  p.  637. 


OCCURRENCE  OF  MONOCLINIC  AMPHIBOLES.  283 

HONOCLIK1C  AMPIIIKOLES. 

The  monoclinic  amphiboles  include  the  following  rock-making  minerals: 

TKKMOUTE,   ACTIXOIJTK,    (TMXIM1TOMTK,    (JRUXERITE,    HORNBLENDE,    ULAl'COPHANj;.    K1EBECKITE,    AND 

ARPVEDSOMTE. 

Tremolite: 

CaMaSi.0,, 

Monoclinic. 
Sp.  gr.  2.9-3.1. 

Actiiiolite: 

Ca(MgFe)3SiA2. 
Monoclinic. 

Sp.  gr.  3-3.2. 
Cummingtonite: 

(MgFe)Si03. 

Monoclinic. 

Sp.  gri  3.1-3.32. 
Grunerite: 

FeSi03. 

Monoclinic. 

Sp.  gr.  3.713. 
Hornblende: 

Chiefly  Ca(MgFe)3Si4O12  with  (MgFe).2(AlFe)4Si2O12,  anti  Na,AljSi«O,,. 

Monoclinic. 

Sp.gr.  3.05-3.47. 
Glcmcophane: 

NaAlSi2O6-(FeMg)SiOs. 

Monoclinic. 

Sp.  gr.  3.103-3.113. 
Siebeckite: 

Na2Fe2Si4O12.FeSiO3. 

Monoclinic. 

Sp.  gr.  3.3. 
Arfoedsonite: 

(Xa2CaFe)4Si4012-(CaMg)2(AlFe)4Si2O12. 

Monoclinic. 

Sp.  gr.  3.44-3.45. 

occurrence. — The  monocliuic  ampliibole  group  of  minerals  is  one  of  the 
most  important  of  the  rock-making  minerals.  Like  the  pyroxenes,  one 
form  or  another  of  ampliibole  may  occur  in  almost  any  kind  of  rock, 
running  from  the  most  basic  to  the  most  acid,  including  both  plutonic  and 
volcanic  rocks,  the  unmodified  sedimentary  rocks,  and  metamorphosed, 
igneous,  and  sedimentary  rocks.  The  amphiboles  develop  extensively  as 
secondary  minerals,  and  especially  is  this  true  for  the  variety  of  ampliibole 
known  as  uralite,  which,  as  seen  on  pp.  274-275,  276-278,  is  derived  from 
corresponding  pyroxenes. 

Tremolite  and  actinolite  are  very  abundant  in  the  schists  metamor- 
phosed from  carbonate  rocks,  especially  those  rich  in  magnesium  and  iron. 


284  A  TREATISE  ON  METAMORPHISM. 

They  also  occur  in  the  metamorphosed  calcareous  fragmental  sediments. 
Where  iron  is  not  abundant,  as  in  the  marbles,  tremolite  is  the  mineral 
which  ordinarily  develops.  Where  ferrous  iron  is  plentiful  actinolite 
normally  forms.  Where  iron  is  the  chief  or  only  carbonate,  griineritc 
ordinarily  develops.  Tremolite  and  actinolite  also  occur  as  alteration 
products  in  igneous  rocks,  being  noted  in  diabases,  gabbros,  and  more  basic 
rocks.  The  secondary  products  frequently  take  the  form  of  asbestos  and 
jade'.  They  are  frequently  associated  with  talc  and  serpentine  hi  steatite- 
schists  or  serpentine-schists.  Often,  also,  tremolite  and  actinolite  are  asso- 
ciated with  pyroxene,  epidote,  and  chlorite.  These  amphiboles  also  occur 
in  veins.  Summarizing,  as  metamorphic  minerals,  tremolite  is  derived  from 
diopside,  dolomite,  and  olivine;  actinolite  from  aukerite,  bronzite,  hyper- 
sthene,  olivine,  parankerite,  and  sahlite. 

Cummingtonite,  the  monoclinic  amphibole  corresponding  in  composi- 
tion with  the  orthorhornbic  amphibole  anthophyllite,  occurs  in  various 
schists  of  metamorphic  origin.  It  is  not  known  as  an  original  constituent 
of  the  igneous  rocks. 

Griinerite  occurs  most  extensively  in  connection  with  magnetite  and 
quartz,  or  with  quartz  alone,  thus  constituting  griinerite-magnetite-quartz- 
schists,  or  griinerite-quartz-schists.  The  grtinerite  in  such  cases  often 
develops  as  a  secondary  product  from  the  alteration  of  siderite,  as  explained 
on  page  245.  Greenalite,  probably  having  the  formula  FeSi03.nH2O,  occurs 
extensively,  as  in  the  Biwabik  formation  of  the  Mesabi  series  of  Minnesota." 
If  such  material  were  so  deeply  buried  as  to  be  altered  under  the  conditions 
of  the  zone  of  anamorphism,  dehydration  would  take  place  and  griinerite 
would  be  formed.  The  mineral  also  occurs  in  the  garnetiferous  micaceous . 
schists;  but  in  some  of  these  rocks  the  griinerite  itself  develops  from  the 
siderite,  as  in  the  case  of  the  pure  griinerite-quartz-schists  and  griinerite- 
magnetite-quartz-schists. 

Hornblende  is  the  most  abundant  of  the  amphiboles,  and  has  a  very 
widespread  occurrence,  being  found  as  a  principal  constituent  in  various 
igneous  rocks,  including  plutonic  and  volcanic  rocks,  and  among  the  latter 
both  in  lavas  and  in  tuffs.  It  also  is  a  constituent  of  some  of  the  sedi- 
mentary rocks.  It  is  a  chief  constituent  of  many  of  the  metamorphosed 

"Leith,  C.  K.,  The  Mesabi  iron-bearing  district  of  Minnesota:  Mon.  U.  S.  Geol.  Survey,  vol.  43, 
1903,  pp.  101-115. 


ALTERATION  OF  MONOCLINIC  AMPHIBOLES.  285 

rocks,  especially  those  of  igneous  origin.  Not  infrequently  it  is  also  an 
abundant  constituent  in  the  metamorphosed  sedimentary  rocks.  The  schists 
in  which  the  hornblende  is  the  chief  constituent,  whether  of  aqueous  or 
igneous  origin,  are  generally  known  as  amphibolites.  In  many  other  schists 
and  gneisses  which  are  chloritic  and  micaceous  it  is  an  important  constitu- 
ent. As  a  metamorphic  mineral  hornblende  has  been  noted  as  derived  from 
almandite,  augite,  melanite,  and  pyrope. 

Glaucophane  occurs  abundantly  in  certain  of  the  amphibole-schists, 
especially  those  which  are  derived  from  the  debris  of  basic  rocks  which  were 
originally  rich  in  sodium.  Naturally,  such  rocks  have  a  somewhat  limited 
occurrence;  but  where  they  do  occur  abundantly,  as  in  the  Coast  Ranges 
.of  California,  glaucophane  is  also  very  abundant — in  fact,  is  the  chief  con- 
stituent of  some  of  the  schists,  so  that  they  may  properly  be  called 
glaucophane-schists. 

Riebeckite  occurs  in  some  eruptive  rocks  which  are  rich  in  sodium  and 
iron,  and  also  in  metamorphosed  rocks  of  both  sedimentary  and  igneous 
origin.  Like  glaucophane,  it  may  locally  occur  abundantly,  but  is  not  a 
widespread  mineral. 

Arfvedsonite,  a  soda-amphibole,  very  naturally  occurs  in  the  soda- 
bearing  igneous  rocks,  especially  in  elseolite-syenites  and  nepheline-syenites. 

Alterations. — The  minerals  of  the  monoclinic  amphibole  group,  ot  such-  a 
wide  variety  of  composition  and  extensive  occurrence,  have  naturally  a 
large  number  of  alteration  products.  The  more  common  of  these  are  talc, 
serpentine,  bastite,  chlorite,  epidote,  and  biotite.  These  are  frequently 
accompanied  by  more  or  less  magnetite,  hematite,  and  limonite.  In  some 
cases  the  amphiboles  alter  into  the  zeolites,  pinite,  and  chabazite. 

Taking  up  the  individual  minerals,  tremolite  is  most  frequently  trans- 
formed into  talc  (orthorhombic  or  monoclinic;  sp.  gr.  2.75),  which  may  be 
accompanied  by  calcite  (rhombohedral ;  sp.  gr.  2.7135).  Accinolite  com- 
monly alters  into  talc  or  serpentine  (monoclinic;  sp.  gr.  2.575),  often 
with  the  simultaneous  formation  of  calcite,  quartz  (rhombohedral;  sp.  gr. 
2.6535),  and  iron  oxide.  Cummingtonite  commonly  alters  to  bastite  (ortho- 
rhombic;  sp.  gr.  2.6).  The  standard  text-books  do  not  describe  the  altera- 
tions of  griinerite,  although  it  is  believed  to  alter  to  the  iron  oxides. 
Hornblende  under  weathering  conditions  ordinarily  changes  to  chlorite 
(monoclinic;  sp.  gr.  2.71-2.725),  which  is  often  accompanied  by  epidote 


286  A  TREATISE  ON  METAMORPHISM. 

(inouoclinic;  sp.  gr.  3.38),  calcite,  quartz,  iron  oxides,  and  siderite  (rhombo- 
hedral;  sp.  gr.  3.855).  Under  deep-seated  conditions  biotite  (monoclinic ; 
sp.  gr.  2.90)  is  frequently  a  product  of  the  alteration  of  hornblende,  and  with 
the  biotite  epidote  may  simultaneously  form.  Rarely  serpentine  is  also  pro- 
duced. While  these  are  the  usual  alterations  of  hornblende,  in  some  cases, 
under  conditions  of  high  temperature,  hornblende  alters  into  augite  (mono- 
clinic;  sp.  gr.  3.4),  with  the  simultaneous  separation  of  magnetite  (isometric; 
sp.  gr.  5.174).  Such  alteration  of  the  hornblende  has  been  noted,  according 
to  Lacroix,  both  in  lavas  and  in  bombs."  In  the  lava  the  change  is  attributed 
to  the  action  of  the  magma,  being  analogous  to  resorption;  but  in  the  bombs 
it  is  attributed  to  heat  alone.  The  alterations  of  glaucophane  and  riebeckite 
are  not  described  in  the  standard  text-books.  Arfvedsonite  under  certain 
conditions  changes  into  an  acmite  (monoclinic;  sp.  gr.  3.525)  free  from 
calcium.6  With  the  acmite  occur  limonite  (amorphous;  sp.  gr.  3.80),  mag- 
netite, and  sometimes  lepidomelane  (monoclinic;  sp.  gr.  3.0-3.2).  The 
change  of  hornblende  ta  augite  and  the  change  of  arfvedsonite  to  acmite 
are  the  reverse  of  the  process  of  uralitization. 

The  alteration  of  tremolite  to  talc  may  be  written  as  follows : 

(1 )     CaMg,Si4Ols+  H,0+COa=H2Mg3Si,Oi.i+CaCOs+k. 

The  increase  in  volume  of  the  talc  and  calcite  as  compared  with  the 
tremolite  is  25.61  per  cent.  The  decrease  in  volume  of  the  talc  alone  as 
compared  with  the  tremolite  is  0.83  per  cent. 

The  alteration  of  actinolite  to  talc,  supposing  the  excess  of  iron  to  sepa- 
rate as  hematite  (rhombohedral ;  sp.  gr.  5.225),  and  supposing  that  the 
Mg :  Fe  : :  2  :  1,  is  as  follows: 

(2)     6CaMg2FeSi1O12+4H3O+6CO2+3O=4H2Mg3Si1O12+6CaCOs+3Fe203+8SiO2+k. 

Supposing  all  the  compounds  to  remain  as  solids  the  increase  in  volume 
is  20.33  per  cent.  If  magnetite  be  formed  instead  of  hematite  the  increase 
in  volume  is  somewhat  less;  if  limonite  be  formed,  considerably  more.  If 
it  be  supposed  that  all  the  compounds  except  the  talc  are  dissolved  the 
decrease  in  volume  is  36.51  per  cent. 

The  alteration  of  actinolite  to  the  variety  of  serpentine  known  as 
bastite  may  be  written  as  follows: 

(3)     Ca(MgFe)sSi4O12+2H2O+CO2=H4(MgFe)?8iA+CaCO,+2SiO2+k. 

a  Lacroix,  A.,  Mineralogie  de  la  France,  Paris,  1893-95,  vol.  1,  pp.  668-669. 
&Br6gger,  W.  C.,  Die  Mineralien  der  Syenitpegniatitgange  der  Siidnorwegischen,  Augit- und 
Nephelin-Syenite:  Zeitschr.  fur  Kryst.  und  Min.,  vol.  16,  1890,  pp.  406-407. 


ALTERATIONS  OF  HORNBLENDE.  287 

Supposing  that  the  Mg  :  Fe  : :  2  :  1,  the  equation  is — 

(4)  CaMg2FeSi4OI2+2H2O+CO2=H4Mg2FeSi2O9+CaCOs-i-2SiO2+k. 

The  increase  in  volume,  supposing  all  the  compounds  to  separate  as  solids, 
is  38.67  per  cent.  If  only  the  bastite  remains  as  a  solid,  the  decrease  in 
volume  is  18.06  per  cent. 

The  alteration  of  cummingtonite  into  bastite  is  as  follows: 

(5)  3(MgFe)Si03+2H20=H4(MgFe)3SiA+Si02+k. 

Supposing  that  the  Mg:  Fe : :  3  : 1,  the  equation  is — 

(6)  3Mg3FeSi4O12+8H2O=H16Mg9Fe3Si8O3644SiO2+k. 

The  increase  in  volume  of  the  bastite  and  quartz  as  compared  with  the 
cummingtonite  is  36.76  per  cent;  of  the  bastite  alone,  14.2  per  cent. 

In  writing  equations  for  the  alteration  of  hornblende  into  chlorite  and 
accompanying  minerals  the  soda-bearing  part  of  the  molecule  will  be 
omitted,  since  the  amount  of  soda  present  in  ordinary  hornblende  is  small. 
Supposing  that  there  are  eight  actinolite  molecules  in  the  hornblende  to 
two  of  the  sesquioxide  molecules,  that  the  MgO  :  FeO  : :  2  : 1,  and  the 
AL203 :  Fe203 : :  2  :  1,  that  the  chlorite  produced  is  on  the  border  line  be.tween 
prochlorite  and  clinochlore,  and  that  the  Al :  Fe  in  the  epidote  as  2  : 1,  the 
reaction  may  be  written  as  follows: 

(7)  8CaMg2FeSi4012.2Mg4Fe2Al8Fe4Si6O36+21H2O+16CO2= 

2(H20Mg12Al6Si7O4.,)+2HCa2Al2FeSisO13+4CaCO3+i2FeC03+24SiO2+3Fe2O3+k. 

Provided  all  the  compounds  separate  as  solids  the  increase  in  volume  is 
25.39  per  cent. 

It  is  noticeable  that  the  equation  for  the  alteration  of  the  hornblende 
to  chlorite  as  a  chief  resultant  product  demands  that  epidote,  calcite,  sid- 
erite,  quartz,  and  hematite  be  produced;  and  corresponding  with  this, 
Lacroix  noted  all  of  these  minerals  as  accompaniments  of  the  chloritic 
alteration  with  the  exception  of  hematite."  Of  course  all  or  a  larger  part 
of  the  iron  may  pass  into  the  form  of  iron  oxide — magnetite,  hematite,  or 
limonite — in  which  case  some  oxygen  would  need  to  be  added  to  the  equa- 
tion; the  amount  of  CO2  required  will  be  less;  and  the  oxide  of  iron  will 
replace  the  iron  carbonate  partly  or  wholly.  As  these  modifications  can 
easily  be  made  in  the  equation,  it  hardly  seems  necessary  to  write  out 
formulae  for  them. 

"Lacroix,  cit.,  pp.  667-668. 


288  A  TREATISE  ON  METAMORPHISM. 

The  change  of  hornblende  to  biotite  requires  the  addition  of  potassium. 
The  potassium  can  be  derived  from  some  other  mineral.  It  is  perhaps 
most,  frequently  derived  from  orthoclase,  although  it  is  undoubtedly  in 
many  cases  derived  from  leucite.  Supposing  it  is  derived  from  orthoclase, 
and  therefore  is  in  the  form  of  potassium  silicate,  K2Si03,  and  that  the 
actinolite  molecule  in  the  hornblende  is  to  the  sesquioxide  molecule  as  2  :  3, 
the  change  may  be  represented  as  follows: 

(8)  2Ca(MgFe)3Si4012-3(MgFe)2(AlFe)4Si2012+aK2Si03+Si02+3H20+2C02= 

6HK(MgFe)2(AlFe),Si,012+2CaCO,+k. 

In  order  to  ascertain  the  volume  relations  it  is  necessary  to  make  assump- 
tions with  reference  to  the  proportions  of  the  Mg  and  Fe,  and  of  the  Al  and 
Fe.  Supposing  the  magnesia  is  to  the  iron  protoxide  as  2:1,  and  the 
alumina  is  to  the  iron  sesquioxide  as  2  :  1,  the  equation  is:— 

(9)  2(CaMg2FeSi4012).Mg1Fe2Al8Fe4Si6036+3K2SiO,+Si02+3H20+2C02= 

H6K6Mg8Fe4Al8Ffe4Si18072+2CaC03+k. 

The  increase  in  volume  of  the  biotite  and  calcite  as  compared  with  the 
hornblende  and  quartz  is  41  13  per  cent.  It  has  been  noted,  however, 
that  tlie  alteration  of  hornblende  to  biotite  is  often  accompanied  by  the 
separation  of  epidote;  and  this  is  natural,  since  there  is  residual  calcium  in 
the  hornblende  not  needed  by  the  biotite,  which  could  pass  into  the 
epidote.  Supposing  this  residual  calcium  to  pass  into  the  epidote,  the 
reaction  may  be  written  as  follows: 


(10)  8Ca(MgFe)3Si4Oi2.18(MgFe)2(AlFe)4Si2O,2 

30HK(MgFe)2(AlFe)2Si3O12  +  4HCa2(AlFe)3Si3O13+k. 

In  order  to  calculate  the  volume  relations  it  may  be  supposed  that  the 
MgO  :  FeO  as  2:1,  and  the  A12O3  :  Fe2O3  as  2:1,  the  equation  being— 

(11)  8CaMg2FeSi4O12.6Mg4Fe.,Al8Fe4Si6O36fl5K2SiO3+19SiO2+17H.!O= 


The  increase  in  volume  of  the  biotite  and  epidote  as  contrasted  with  the 
hornblende  and  quartz  is  30.05  per  cent.  The  increase  in  volume  for 
equations  (9)  and  (11)  would  be  much  less  if  the  K2SiO3  were  taken  into 
account, 

Where  serpentine  also  occurs  as  an  alteration  product  accompanying 
the  chlorite,  biotite,  epidote,  and  other  products,  this  mineral  is  doubtless 
derived  from  the  actinolite  part  of  the  molecule,  and  an  equation  may  be 
readily  written  which  represents  the  simultaneous  formation  of  the  bastitic 


ALTERATIONS  OF  HORNBLENDE.  289 

form  of  serpentine  by  supposing  that  the  number  of  actinolite  molecules  is 
greater  than  given  in  the  above  equations  and  that  such  excess  of  these 
molecules  passes  into  bastite,  according  to  equation  (3). 

While  the  more  common  alterations  of  hornblende  are  to  chlorite, 
biotite,  epidote,  and  accompanying  minerals,  as  above  explained,  the  change 
of  hornblende  into  augite,  just  the  reverse  of  that  of  augite  into  hornblende 
described  on  pages  274-278,  does  take  place,  and  probably  on  a  great  scale 
at  sufficient  depth.  .The  equation  for  one  case  may  therefore  be  written: 

(12)  Ca2MgsFe3SiA4.  (MgFe)2(AlFe)<Si2O12+2CaCOs= 

2[Ca2MgFeSi4O,2.  ( MgFe)  ( AlFe)  2SiO6]  +FeC05+ MgCO3+k. 

If  the  Mg :  Fe  : :  2  : 1,  and  the  Al :  Fe  : :  2  : 1,  the  equation  is — 

(13)  Ca2Mg3Fe3Si8024-Mg,Fe2Al8Fe4Si6O36+2CaCO3= 

2[Ca2MgFeSi4012.  Mg2FeAl4Fe2Si3018] +FeC03+MgCO,+k. 

The  decrease  in  volume  of  the  augite  as  compared  with  the  amphibole  is 
4.13  per  cent. 

It  is  not  supposed  that  the  above  equations  for  the  alteration  of  horn- 
blende necessarily  represent  the  actual  facts  of  specific  cases.  Doubtless  in 
most  instances  materials  from  minerals  aside  from  those  given  enter  into 
the  alterations,  and  the  actual  changes  are  more  complex  than  represented. 
However,  the  equations  very  clearly  show  why  it  is  that  the  production  of 
chlorite  from  hornblende  demands  also  the  production  of  other  minerals 
which  Lacroix  says  so  generally  accompany  chlorite.  Also,  they  show  why 
epidote  so  frequently  accompanies  biotite  secondary  to  hornblende.  The 
equations  may  be  considered  as  average  cases,  which  approximate  to  the 
alterations  that  actually  occur  in  many  instances.  The  volume  relations 
calculated  from  the  equations  also  are  probably  averages,  for  the  proportions 
of  the  elements  taken  in  the  equations  given  are  chosen  from  a  considera- 
tion of  analyses  of  the  various  minerals.  The  equations  at  least  make  a 
quantitative  estimate  of  the  relations  of  the  original  and  secondary  minerals, 
and  therefore  will  lead  to  closer  observations  as  to  the  minerals  which 
result  from  the  alteration  of  hornblende,  and  their  relative  proportions. 

The  change  of  arfvedsonite  into  acmite  is  so  uncertain  in  its  character 
that  no  attempt  is  made  to  write  out  the  equations.  In  order  to  satisfactorily 
write  equations  for  this  alteration  it  is  necessary  to  know  the  composi- 
tion of  the  particular  arfvedsonite  which  changes  into  the  particular  acmite, 
and  what  other  minerals  aside  from  the  acmite  are  produced  in  the  change. 
MON  XLVII — 04 19 


290  A  TREATISE  ON  METAMORPHISM. 

The  alteration  of  tremolite  to  talc,  equation  (1),  is  that  of  hydration 
and  carbonation.  The  alteration  of  actinolite  to  talc,  equation  (2),  is  that 
of  hydration,  carbouation,  desilication,  and  oxidation.  The  alteration  of 
actinolite  to  bastite,  equations  (3)  and  (4),  is  that  of  hydration,  carbonation, 
and  desilication.  The  alteration  of  cuinmingtonite  to  bastite,  equations  (5) 
and  (6),  is  that  of  hydration  and  desilication.  All  these  changes  take  place 
with  the  liberation  of  heat  and  with  expansion  of  volume,  provided  the 
compounds  which  form  mainly  separate  as  solids.  Whether  or  not  there  is 
an  actual  increase  in  the  volume  as  a  result  of  the  changes  depends,  of 
course,  upon  the  amounts  of  the  secondary  material  which  is  dissolved.  It 
is  therefore  clear  that  all  of  these  changes  are  those  which  are  typical  of  the 
zone  of  katamorphism,  and  especially  the  belt  of  weathering.  Moreover, 
some  of  the  changes,  like  that  of  actiuolite  to  talc  and  the  accompanying 
compounds,  illustrate  all  the  processes  normal  to  this  position;  i.  e.,  hydration, 
carbonation,  oxidation,  and  desilication.  The  fact  that  calcite  is  so 
frequently  found  associated  with  the  talcs  and  serpentines  secondary  to 
tremolite,  cummingtonite,  and  actinolite,  is  rendered  perfectly  clear  by  the 
equations;  for  there  is  always  a  residuum  of  calcium  which  evidently,  under 
the  conditions  of  the  upper  physical-chemical  zone,  unites  with  the  carbon 
dioxide  and  produces  calcium  carbonate,  which  frequently  separates  as  the 
mineral  calcite  in  large  part,  but  which  doubtless  is  frequently  largely  or 
altogether  carried  away  in  solution. 

The  alteration  of  hornblende  into  chlorite  and  accompanying  minerals 
is  one  of  liberation  of  heat  and  expansion  of  volume.  It  is  an  alteration 
also  of  carbouation,  and  of  oxidation  in  case  some  of  the  ferrous  iron  be 
changed  to  sesquioxide.  It  is  therefore  to  be  expected  in  the  upper 
physical-chemical  zone,  and  as  a  matter  of  fact  it  occurs  there.  The  change 
from  hornblende  to  biotite  is  a  much  deeper  seated  alteration.  It  involves 
hydration,  silication,  and  possibly  earbonation,  and  thus  includes  an 
unusual  combination  of  reactions.  Corresponding  with  these  facts  the 
change  of  hornblende  to  biotite  is  one  which  takes  place  under  rather 
deep-seated  conditions,  particularly  in  connection  with  profound  mechan- 
ical action.  The  physics  of  the  interchanges  between  hornblende  and 
augite  are  elsewhere  discussed  (see  pp.  279-280);  but  it  may  be  said  that 
the  change  of  the  first  to  the  second  involves  decrease  of  volume,  and, 
corresponding  with  this  fact,  is  known  to  take  place  under  very  deep- 
seated  conditions  of  metamorphism. 


OCCURRENCE  AND  ALTERATION  OF  IOLITE.  291 

IOLITE  (CORDIERITE). 

lolite  (cordierite)  : 

H2(MgFe)4Al8Si10037. 
Orthorhonibic. 
Sp.  gr.  2.60-2.66. 

occurrence. — lolite  occurs  in  a  great  variety  of  schists  and  gneisses.  In 
some  cases  it  is  so  abundant  as  to  make  the  rock  a  cordierite-gneiss.  It  is 
associated  with  the  very  heavy  metamorphic  minerals,  such  as  tourmaline, 
andalusite,  sillirnanite,  garnet,  etc.  lolite  occurs,  likewise,  in  ejected  frag- 
ments of  volcanoes  and  as  a  contact  mineral  in  connection  with  dikes ;  also 
rarely  as  an  original  mineral  in  igneous  rocks. 

Alterations. — The  most  common  alteration  is  simple  hydration.  Further 
changes  may  remove  some  of  the  ferrous  iron  or  introduce  alkalies,  or 
both,  forming  pinite  (massive;  sp.  gr.  2.775).  Simultaneously  with  this  an 
isotropic  substance  is  said  to  be  formed.  lolite  sometimes  passes  into  a 
chlorite  similar  to  talc. 

By  the  hydration  of  iolite,  according  to  Clarke,  chlorophyllite  (crystal- 
lization not  given;  sp.  gr.  2.77)  is  formed.0  Supposing  the  Mg  and  Fe  to 
be  in  the  same  proportions  both  in  the  iolite  and  in  the  chlorophyllite,  the 
reaction  is  simple: 

(1)  H2(MgFe)tAl8Si10037+3H.20  =  H8(MgFe)1Al8Si1001o+k. 

If  it  be  supposed  that  the  Mg :  Fe  : :  3  :  1  in  both  compounds,  the  equation  is — 

(2)  H2Mg3FeAl8Si10031-i-3H20=H8Mg3FeAl8Si10010+k. 

The  decrease  in  volume  is  0.86  per  cent. 

The  reaction  being  hydration,  one  would  expect  it  to  involve  increase 
of  volume,  but  the  chlorophyllite  produced  is  enough  heavier  to  compensate 
for  this.  One  would  expect  the  reaction  to  take  place  in  the  zone  of  kata- 
morphism,  but  observations  on  this  point  are  not  known  to  me. 

The  character  of  the  product  which  forms  simultaneously  with  pinite 
being  unknown,  and  the  character  of  the  chlorite  which  forms  as  a  sec- 
ondary product  not  being  ascertained,  it  seems  hardly  worth  while  to 
attempt  to  write  equations  for  these  alterations,  for  they  would  be  largely 
conjectural. 

"Clarke,  F.  W.,  The  Constitution  of  the  silicates:  Bull.  U.  S.  Geol.  Survey  No.  125,  1895,  p.  83. 


292  A  TREATISE  ON  METAMORPHISM. 


NEPHELITE    GROUP. 
XEPHELITE  AJiD  t'AXCBIMTE. 

The  nephelite  group  includes — 

Nephelite: 

NaAlSiO4. 

Hexagonal. 

Sp.  gr.  2.55-2.65. 

Cancrinite : 

H6NasCa  (NaCO,),Al8Si,OM. 

Hexagonal. 

Sp.  gr.  2.42-2.50. 

NEPHELITE. 

occurrence. — Nephelite  is  a  sodium-aluminum  silicate.  Commonly  the 
sodium  is  in  part  replaced  by  potassium.  Nephelite  occurs  in  both  ancient 
and  modem  igneous  rocks,  both  surface  and  deep  seated.  It  is  abund- 
ant in  the  syenite-schists  and  syenite-gneisses  of  certain  localities,  but 
is  not  known  in  the  metamorphosed  secondary  rocks.  This  is  doubtless 
due  to  its  ready  alteration.  Nephelite  has  been  produced  artificially  at 
220°  C.  by  a  reaction  between  kaolinite  and  an  alkaline  carbonate.  As  a 
secondary  product  nephelite  forms  from  leucite,  but  this  alteration  is  not 
an  important  source  of  the  mineral.  Nephelite  is  al(-o  probably  derived 
from  sodalite. 

Alterations. — The  most  frequently  observed  alteration  of  nephelite  is  to 
the  zeolites,  and  especially  to  hydronephelite  (hexagonal;  sp.  gr.  2.263), 
natrolite  (orthorhombic;  sp.  gr.  2.20-2.25),  thomsonite  (orthorhombic;  sp. 
gr.  2.3—2.4),  and  analcite  (isometric;  sp.  gr.  2.22-2.29).  Simultaneously 
with  the  formation  of  some  of  the  zeolites  diaspore  (orthorhombic;  sp.  gr. 
3.3-3.5),  or  gibbsite  (monoclinic;  sp.  gr.  2.35),  or  kaolinite  (monoclinic;  sp. 
gr.  2.615),  or  some  combination  of  these,  is  frequently  formed. 

The  reaction  for  hydronephelite  is — 

(1)  6NaAlSiO4+7H2O+CO,=2(HNa.!AlsSisO12.3H.!O)  -t-Na-jCOa+k. 

The  increase  in  volume  is  23.49  per  cent. 

The  alteration  next  in  importance  is  to  natrolite  and  gibbsite,  or  to 
natrolite  and  diaspore.  The  reaction  in  the  former  case  is: 

(2)  6NaAlSi04+7H20+C01=2Na2Al2H4Si80I2+2Al  (OH)s+Na,COs+k. 


ALTERATIONS  OF  NEPHELITE.  293 

Supposing-  the  sodium  carbonate  to  be  earned  off  in  solution,  the  increase 
in  volume  would  be  24.4G  per  cent.  If  two  molecules  less  of  water  were 
added,  instead  of  two  molecules  of  gibbsite,  two  molecules  of  diaspore 
would  be  formed,  according  to  the  reaction: 

(3)  6XaAlSi01+5H2O+C02=2Na2Al2H1SisO12+2AlO(OH)+Na2COs+k. 

In  this  case  the  increase  in  volume  would  be  only  15  per  cent. 

In  the  production  of  thomsonite,  calcium  must  replace  the  sodium.  It 
will  be  assumed  that  this  calcium  is  derived  from  calcium  carbonate.  The 
reaction  will  then  be — 

(4)  6NaAlSiO4H-7H2O+3CaCO3=Ca3Al6Si6O24.7H,O+r-iNa2CO3+k. 

Supposing  the  calcium  carbonate  to  have  been  brought  in  solution  and  the 
sodium  carbonate  carried  away  in  solution,  the  increase  in  volume  is  24.60 
per  cent. 

The  less  common  alteration  of  nephelite  to  the  zeolite  analcite,  with 
the  simultaneous  production  of  diaspore  or  gibbsite,  is  expressed  by  the 
following  reactions: 

(5)  4NaAlSiO4+3H2O+CO2=NaJAl2Si1012.2H2O+2[AlO(OH)]+Na2CO,+k 
or 

(6)  4NaAlSi04+5H2O+CO2=Na2Al2SiA^2H2O+2Al(OH)s+Na2COs+k. 

In  the  first  case  diaspore  is  simultaneously  formed,  and  in  the  second  case 
gibbsite.  Supposing  the  sodium  carbonate  to  be  carried  away  in  solution 
the  increase  in  volume  is  5.49  per  cent  if  diaspore  be  formed,  and  19.68 
per  cent  if  gibbsite  be  formed. 

Alterations  of  nephelite  to  muscovite  (monoclinic;  sp.  gr.  2. 88),  to  hydro- 
muscovite  (pinite)  (massive;  sp.  gr.  2.775),  and  to  kaolinite  (monoclinic;  sp. 
gr.  2.6-2.63)  have  also  been  noticed.  Where  this  alteration  takes  place 
the  nephelite  is  probably  a  potassium-bearing  one.  Assuming  that  the 
amount  of  potassium  is  one-third  of  the  sodium,  the  reaction  may  be  written: 

(7)  2KNasAl4Si4OI8+4H2O+3002=2KH2AlsSi8O12+H1Al2Si2O,+  3NaJCO3+k. 

The  decrease  in  volume  of  the  muscovite  and  kaolin  as  compared  with  the 
nephelite  is  16.50,  provided  the  sodium  carbonate  is  carried  away  in  solu- 
tion. The  decrease  is  13  per  cent  if  the  products  are  pinite  and  kaolinite. 
The  volume  of  the  muscovite  alone  is  38.46  per  cent  less  than  that  of  the 
nephelite. 


294  A  TREATISE  ON  METAMORPHISM. 

Another  alteration  of  nephelite  of  some  importance  is  to  sodalite 
(isometric;  sp.  gr.  2.14-2.30) — 

(8)  3NaAlSi04-j-NaCl=XaC1.3NaAlSi04+k. 

Supposing  the  NaCl  to  be  added  in  solution,  the  increase  in  volume  is 
33.14  per  cent.  If  the  sodium  chloride  be  present  as  solid  halite  (isometric; 
sp.  gr.  2.1-2.6),  the  increase  in  volume  would  be  15.64  per  cent. 

While  the  change  is  not  recorded,  it  is  believed  to  be  highly  probable 
that  nephelite  during  mass  deformation  under  deep-seated  conditions  may 
change  into  feldspar,  probably  albite  (triclinic;  sp.  gr.  2.62-2.65).  This 
reaction  would  require  the  addition  of  silica,  as  follows: 

(9)  NaAlSi04+2SiO2=NaAlSisO8+k. 

Supposing  the  silica  to  have  been  present  as  quartz  (rhombohedral ;  sp.  gr. 
2.653-2.654),  the  decrease  in  volume  would  be  0.41  per  cent. 

The  formation  of  the  zeolites,  and  simultaneously  the  minerals  gibbsite 
or  diaspore,  equations  (1)  to  (6),  are  all  alterations  of  hydration,  carbona- 
tion,  and  expansion  of  volume,  except  that  of  thomsouite,  equation  (4), 
which  does  not  involve  carbonation.  It  is  therefore  to  be  expected  that 
these  are  reactions  which  take  place  in  the  zone  of  katamorphism,  and  such 
is  the  fact.  As  a  result  of  the  alteration  of  the  nephelites  to  the  zeolites  in 
this  zone,  a  part  of  the  sodium  separates  and  probably  goes  into  solution  as 
sodium  carbonate,  and  thus  we  have  one  of  the  sources  of  this  compound 
which  so  frequently  occurs  in  underground  waters,  especially  in  volcanic 
regions.  The  formation  of  muscovite  and  kaolinite  from  nephelite  is  a 
reaction  involving  hydration  and  carbonation  and  decrease  of  volume,  and 
therefore  is  characteristic  of  the  zone  of  katamorphism.  The  formation  of 
sodalite  from  nephelite  is  one  which  might  take  place  in  either  physical- 
chemical  zone,  only  in  the  upper  zone  the  sodium  chloride  would  probably 
be  added  in  solution,  while  in  the  lower  zone  it  would  pi'obably  be  derived 
from  solid  halite. 


CANCRIXITE. 


occurrence. — Cancrinite  is  known  only  in  the  nepheline  syenites. 

Alterations. — By  Dana  it  is  mentioned  as  altering  to  natrolite  (orthorhombic ; 
sp.  gr.,  2.225).  The  reaction,  supposing  the  excess  of  alumina  passes  into 
gibbsite  (monoclinic;  sp.  gr.,  2.35),  may  be  as  follows: 


)  2Al8Si9O36+6HsO= 
3(Na5Al2Si3010.2H.iO)+2Al(OH),>4CaCO,+Na.(COs+k. 


OCCURRENCE  AND  ALTERATIONS  OF  SODALITE.      295 

The  increase  in  volume  of  the  natrolite,  gibbsite,  and  calcite  (rhombo- 
hedral;  sp.  gi-.,  2.7135)  as  compared  with  cancrinite  is  8.64  per  cent. 
The  reaction  is  that  of  hydration  and  breaking-  up  of  a  complex  com- 
pound into  several  simpler  compounds  requiring  greater  volume,  and  is 
therefore  typical  of  the  zone  of  katamorphism. 

SODALITE   GROUP. 
KODALITK,  HAl'VMTE,  AND  >OSELITE. 

The  socialite  group  includes — 

Sodalite: 

XaCUXaAlSiO,. 
Isometric. 

Sp.  gr.,  2.14-2.30. 

Hauyniie: 

Xa.,Ca(XaSO4.  Al)  Al2Si30,2. 

Isometric. 

Sp.  gr.,  2.4-2.5. 

Noselite: 

Na4(NaS04.Al)Al2Si3012. 

Isometric. 

Sp.  gr.,  2.25-2.4. 

SODALITE. 

occurrence. — Socialite  is  sodium  aluminum  silicate  with  some  chloride. 
Socialite  occurs  as  an  original  constituent  in  the  igneous  rocks,  both  surface 
and  deep  seated.  It  is  not  known  in  the  secondary  rocks  or  their  metamor- 
phosed equivalents.  In  fact,  the  occurrence  of  sodalite  is  almost  identical 
with  that  of  nephelite,  which  mineral  is  one  of  its  sources. 

Alterations. — The  alteration  products  of  sodalite  are  also  identical  with 
those  of  nephelite,  except  that  nephelite  passes  into  sodalite,  and  the 
reverse  reaction  is  not  recorded,  although,  as  noted  below,  it  is  believed  to 
occur.  The  alterations  of  sodalite  into  minerals  similar  to  those  into 
which  nephelite  alters  is  natural,  as  sodalite  is  made  up  of  the  nephelite 
molecule  with  the  addition  of  sodium  chloride. 

Sodalite  alters  to  the  same  zeolites  as  does  nephelite,  viz,  to  hydro- 
nephelite  (hexagonal;  sp.  gr.,  2.263),  natrolite  (orthorhombic ;  sp.  gr.,  2.2- 
2.25),  thomsonite  (orthorhombic;  sp.  gr.,  2.3-2.4),  and  analcite  (isometric; 
sp.  gr.,  2.22-2.29).  Simultaneously  with  the  formation  of  some  of  the 


296  A  TREATISE  ON  METAMORPHISM. 

zeolites,  diaspore  (orthorhombic ;  sp.  gr.,  3.3-3.5)  or  gibbsite  (monoclinic; 
sp.  gr.,  2.3-2.4)  is  frequently  formed. 

In  the  production  of  hydronephelite  the  reaction  is — 

(1)  2(NaC1.3NaAlSiO4)+4H2O+CO2=2HNa,Al3Si3O12.3H2O+2NaCl+Na,CO,+k. 

Supposing  that  the  sodium  chloride  and  sodium  carbonate  are  dissolved, 
the  decrease  in  volume  is  7.25  per  cent. 

In  the  alteration  of  sodalite  to  natrolite,  gibbsite  or  diaspore  is  also 
produced.     The  reaction,  provided  gibbsite  be  produced,  is — 

(2)  2(NaC1.3NaAlSi04)+7H20+C02=2Na2Al9H4Si801J+2Al(OH)3+2NaCl+Na2COs+k. 

If  two  molecules  less  of  water  were  added,  in  place  of  the  gibbsite  two 
molecules  of  diaspore  would  be  produced,  according  to  the  reaction: 

(3)  2(NaC1.3NaAlSiO4)-t-5H.iO+CO.!=2Na.!Al2H4Si3Ola-(-2AlO(OH)+2NaCl+Na.!COs+k. 

Supposing  the  sodium  chloride  and  sodium  carbonate  to  be  dissolved, 
the  decrease  in  volume  in  the  first  case  would  be  6.52  per  cent,  and  in  the 
second  case  13.62  per  cent. 

In  the  production  of  thomsonite  the  reaction  is — 

(4)  2(NaC1.3NaAlSiO4)+7H2O+3paCO3=CasAl6Si6O24.7H2O+2NaCl-t-3Na2CO3+k. 

Supposing  the  calcium  carbonate  to  have  been  in  solution  and  the  sodium 
chloride  and  sodium  carbonate  to  be  taken  into  solution,  the  decrease  in 
volume  is  6.41  per  cent. 

The  alteration  of  sodalite  to  analcite  and  to  diaspore  may  be  written 
as  follows: 

(5)  4(NaC1.3NaAlSiO4)  +9H2O+3CO2= 

3(Na.,Al2Si4O12.2H8O) +6A1O(  OH )  +4NaCl+3(  Na2COs)  +k. 

If  six  additional  molecules  of  water  were  added,  as  in  the  case  of  the 
reaction  written  for  natrolite,  gibbsite  instead  of  diaspore  would  be  formed. 
The  reaction  is — 

(6)  4(NaC1.3NaAlSiO4)+15H2O+3CO2= 

3(Na,Al2Si4012.2H2O)+6Al(OH)3+4NaCl+3Na.!C03+k. 

Supposing  the  sodium  chloride  and  sodium  carbonate  to  be  taken  into 
solution,  the  decrease  in  volume  is  20.77  per  cent  in  the  case  of  diaspore 
and  10.11  per  cent  in  the  case  of  gibbsite. 


OCCURRENCE  OF  HAUYNITE  AND  NOSELITE.  297 

The  reaction  for  the  alteration  of  sodalite  to  muscovite  (monoclinic; 
sp.  gr.,  2.76-3)  and  kaolinite  (monoclinic;  sp,  gr.,  2.6-2.63),  supposing 
potassium  to  replace  one-fourth  of  the  sodium  of  the  silicate,  would  be — 

(7)     2(4NaCl.K3Na9Al12  Si12O48)+12H2O+9CO2= 

6KH2Al3Si3O,,+3H1Al2Si2O!)+8NaCl+9Na!CO3+k. 

Provided  the  sodium  chloride  and  sodium  carbonate  are  dissolved,  the 
decrease  in  volume  is  37.07  per  cent. 

As  in  the  case  of  nephelite,  it  is  suspected  that  sodalite  may  pass  into 
albite  or  other  feldspar.  However,  as  this  chang-e  is  conjectural,  no  reaction 
will  be  written. 

The  various  reactions  above  given  are  analogous,  both  from, a  physical- 
chemical  point  of  view  and  from  a  geological  point  of  view,  with  the  corre- 
sponding reactions  in  the  case  of  nephelite.  Hence  it  need  only  be  said 
that  the  changes  written  are  those  occurring  in  the  zone  of  katamorphism, 
in  which  rock  fracture  occurs  and  ground  solutions  are  active.  These 
ground  solutions  by  the  changes  become  bearers  of  sodium  chloride  and 
sodium  carbonate. 

The  relations  between  the  alterations  of  nephelite  and  sodalite  illustrate 
very  well  the  law  of  mass  action.  In  the  laboratory,  if  nephelite  be  exposed 
to  the  "slow  action  of  fused  sodium  chloride  with  the  addition  of  vaporized 
Nad"  it  is  changed  into  sodalite."  On  the  contrary,  however,  in  nature, 
where  water  is  abundant  and  the  amount  of  sodium  chloride  is  small,  the 
reverse  reaction  takes  place,  and  sodium  chloride  is  abstracted.  Probably 
at  the  same  time  the  nephelite  molecule  is  altered  as  above  indicated. 
Thus,  while  observation  does  not  as  yet  record  nephelite  as  an  alteration 
product  of  sodalite,  it  is  believed  to  be  highly  probable  that  this  mineral  is 
really  formed  as  a  stage  in  the  process  of  alteration  of  sodalite. 

HAttYNITE    AXD    NOSELITE. 

occurrence. — Haiiyiiite  is  sodium-calcium-aluminum  silicate  with  some 
sulphate.  Noselite  is  sodium-aluminum  silicate  with  some  sulphate. 

"Dana,  J.  D.,  A  system  of  mineralogy;  Descriptive  mineralogy,  by  E.  S.  Dana,  Wiley  &  Sons,  New 
York,  6th  ed.,  1892,  p.  430.    See  also  Rosenbusch,  Mikroskopische  Physiographic,  Stuttgart,  1885,  p.  284. 


298  A  TREATISE  ON  METAMORPHISM. 

Haiiynite  and  noselite  are  common  in  certain  igneous  rocks,  especially 
those  which  contain  nephelite  and  leucite.  Neither  of  these  minerals  is 
known  in  the  schists  and  gneisses  derived  from  the  sedimentary  rocks. 

Alterations.  —  The  minerals  alter  to  zeolites,  especially  to  natrolite  (ortho- 
rhombic;  sp.  gr.  2.20-2.25),  stilbite  (monoclinic  ;  sp.  gr.  2.094-2.205),  and 
chabazite  (rhombohedral  ;  sp.  gr.  2.08-2.16).  Simultaneously  with  certain 
of  these  alterations  calcite  (rhombohedral;  sp.  gr.  2.713-2.714)  also  forms. 

Noselite  passes  into  natrolite  according  to  the  following  reaction  : 

(1)  2Na4(NaSO«.Al)Al,Si,0ls+CO,+7H,O=- 

2(H4Na2Al2Si,012)+2Al(OH)3+2Na2S04+Na,C03+k. 

It  appears  that  the  change  requires  the  formation  of  gibbsite  (mono- 
clinic;  sp.  gr.  2.3-2.4)  or  diaspore  (orthorhombic;  sp.  gr.  3.3-3.5),  although 
these  minerals  are  not  recorded  as  forming  contemporaneously  with  the 
natrolite.  Supposing  the  gibbsite  to  separate  as  a  solid,  and  the  sodium 
sulphate  and  sodium  carbonate  to  be  taken  into  solution,  the  decrease  in 
volume  is  16.44  per  cent. 

The  parallel  reaction  for  the  passage  of  haiiynite  into  natrolite  and 
gibbsite  is  as  follows: 

(2)  2Na,Ca(NaSO4.  Al)  Al2Si3O12+2CO2+8H2O= 

2(H4Na,Al2Si3O,2)+2Al(OH)s+2CaCO3+2NaHSO4+k. 

Supposing  the  gibbsite  and  calcite  to  remain  as  solids  with  the  natrolite,  but 
the  sodium  acid  sulphate  to  pass  into  solution,  the  increase  in  volume  is  4.99 
per  cent. 

As  stilbite  is  a  calcium-bearing  silicate,  it  may  be  assumed  that  this 
forms  from  haiiynite  rather  than  noselite.  The  reaction  is  as  follows: 

(3)  6Na2Ca(NaSO4.Al)Al.!Si3O12+36H2O+6CO2= 


It  appears  that  the  reaction  for  the  formation  of  stilbite  thus  requires  the 
formation  of  calcite,  and  also  of  gibbsite  or  diaspore.  The  equation  is 
written  for  the  former  mineral,  but  could  readily  be  changed  to  the  latter. 
Supposing  the  calcium  carbonate  and  the  gibbsite,  as  well  as  the  stilbite,  to 
be  solids,  and  the  other  compounds  to  be  taken  into  solution,  the  increase 
in  volume  is  0.460  per  cent. 


MINERALS  OF  GARNET  GROUP.  299 

The  reaction  for  the  formation  of  chabazite  from  haiiynite  is — 

(4)     4Na,Ca(NaS04.Al)Al2Si3012+24H2O+6CO2= 

Ca3Al6(Si04)3(Si308)3-18H20+4Al(OH)3+CaS04+Al2(S01)3+6Na.!C03+k. 

This  reaction  again  requires  the  formation  of  gibbsite  or  diaspore.  Sup- 
posing the  compound  to  be  gibbsite,  and  it  and  the  chabazite  to  remain  as 
solids,  and  the  other  compounds  to  be  taken  into  solution,  the  decrease  in 
volume  is  7.46  per  cent. 

The  alterations  of  haiiynite  and  noselite  to  the  zeolites,  calcite,  and 
gibbsite  or  diaspore  are  all  reactions  of  hydration  and  carbonation  and 
liberation  of  heat.  If  the  readily  soluble  compounds  are  dissolved,  as  is 
probable,  the  volume  is  decreased  in  most  instances.  The  reactions  are 
therefore  characteristic  of  the  zone  of  katamorphism. 

GAKNET   GROUP. 
(JROSSULARITE,   PYROPE,  ALMAXDITK,  SPESSARTITE,  MELAXITE,  AXD  UVABOTITE. 

The  garnet  group  includes  the  following  rock-making  species: 

Grossularite: 
Ca3Al.2Si3O12. 
Isometric. 
Sp.  gr.  3.55-3.66. 

Pyrope: 

Mg3Al2Sis012. 

Isometric. 

Sp.  gr.  3.70-3.75. 

Almandite: 

Fe3Al2Si3OI2. 
Isometric. 
Sp.  gr.  3.9-4.2. 

Spessartite: 

Mn3Al2Si30,.,. 

Isometric. 

Sp.  gr.  4.00-4.30. 

Melanite: 

Ca3Fe2Si3O12. 

Isometric. 

Sp.  gr.  3.80-3.90. 

Uvarovite: 

Ca3O2Si3O12. 

Isometric. 

Sp.  gr.  3.41-3.52. 


300  A  TREATISE  ON  METAMORPHISM. 

occurrence. — Some  form  of  garnet  is  a  very  common  mineral  in  a  great 
variety  of  the  schists  and  gneisses,  including  those  which  are  derived  from 
sediments  and  from  all  forms  of  igneous  rocks,  plutonic  and  volcanic,  both 
lavas  and  tuffs.  Ordinarily  the  garnet  is  a  subordinate  constituent  in  these 
rocks,  although  in  some  cases  it  becomes  one  of  the  chief  constituents.  The 
mineral  has  its  most  widespread  occurrence  in  the  metamorphosed  rocks 
which  have  altered  under  the  influence  of  mechanical  action,  or  with  the 
assistance  of  igneous  injections,  or  both.  Not  infrequently  where  garnet  is 
particularly  abundant  combined  contact  and  mechanical  action  have  assisted 
in  furnishing  the  conditions  favorable  to  its  formation.  In  many  instances 
the  garnet  develops  after  the  mechanical  action  has  ceased,  showing  that  it 
was  not  the  movements  themselves  but  the  other  favorable  conditions  result- 
ing therefrom  which  produced  the  garnets.  It  appears,  therefore,  that  the 
conditions  favorable  for  the  extensive  development  of  the  mineral  are  heat, 
moisture,  and  high  pressure.  The  mineral  garnet  is  the  most  important  of 
a  group  of  heavy  metamorphic  minerals  which  form  under  the  conditions 
mentioned.  Other  minerals  which  form  under  similar  conditions  and  are 
frequently  associated  with  garnet  are  wollastonite,  cordierite,  vesuvianite, 
scapolite,  chondrodite,  staurolite,  andalusite,  sillimanite,  cyauite,  tourmaline, 
zircon,  etc.  These  minerals  are  all  anhydrous,  or  nearly  so,  and  mostly 
of  a  high  specific  gravity,  many  of  them  having  a  high  symmetry.  All  of 
them  are  formed  by  the  union  of  silica  with  bases,  and  are  therefore 
produced  by  processes  of  silication.  In  many  instances  this  simultaneously 
involves  decarbonation,  and  this  change,  as  already  explained,  p.  177, 
absorbs  heat  and  lessens  the  volume  of  the  compounds.  They  are 
therefore  minerals  which  form  normally  in  the  zone  of  anamorphism. 

Garnet  thus  produced  can  not  in  general  be  said  to  have  been  derived 
from  any  single  mineral.  It  is  usually  the  result  of  the  rearrangement  of 
material  of  two  or  more  adjacent  minerals.  Dana  notes  °  that  when  garnet 
.is  fused,  and  the  material  recrystallizes,  the  resultant  minerals  are  usually 
pyroxene,  melilite,  monticellite,  scapolite,  anorthite,  nephelite,  etc. 

This  doubtless  gives  an  indication  as  to  some  of  the  minerals  which 
are  rearranged  under  the  conditions  above  described  for  the  development  of 
garnet,  which  are  very  different  from  those  of  dry  fusion.  Also  it  is 

«  Dana,  J.  D.,  A  system  of  mineralogy;  Descriptive  mineralogy,  by  E.  8.  Dana,  Wiley  &  Sons,  New 
York,  6th  ed.,  1892,  p.  447. 


MINERALS  OF  GARNET  GROUP.  301 

certain  that  various  hydrous  minerals  furnish  material  for  the  formation  of 
garnet,  and  also  the  limestones  and  dolomites.  As  already  noted,  garnet 
has  a  great  variation  in  composition,  and  in  a  given  case  one  of  the  pure 
species  mentioned,  or  a  combination  of  the  molecules  of  two  or  more  of 
them,  will  be  formed  which  can  be  derived  from  the  elements  available. 
For  instance,  from  an  impure  limestone,  calcium-aluminum  garnet,  grossu- 
larite,  is  likely  to  form.  In  the  magnesian  rocks,  magnesium-aluminum 
garnet,  pyrope,  is  likely  to  be  produced.  In  the  impure  aluminous 
carbonates  of  calcium,  magnesium,  and  iron,  some  combination  of  two  or 
more  of  the  species  grossularite,  pyrope,  almandite,  and  melanite  is  likely 
to  be  produced. 

Garnet  may  be  an  original  constituent  of  some  of  the  igneous  rocks. 
If  this  be  so,  this  source  of  garnet  is  comparatively  insignificant,  as  it  is 
very  rare  indeed  that  garnet  is  found  in  an  unaltered  igneous  rock.  In 
some  of  the  little  altered  igneous  rocks  it  is  found  in  lithophysae,  but  the 
garnets  in  this  position  are  apparently  the  latest  products  of  crystallization, 
the  conditions  of  their  formation  being  analogous  to  those  producing  garnets 
under  the  ordinary  conditions  of  rock  metamorphism. 

Considering  the  garnets  individually,  the  following  statements  can  be 
made  as  to  their  occurrence: 

Grossularite  is  especially  common  in  the  marbles,  where  it  is  frequently 
associated  with  vesuvianite,  wollastonite,  diopside,  etc.  It  also  occurs  in 
the  calcareous  schists  and  gneisses,  especially  in  the  calcareous  siliceous 
rocks,  such  as  calcareous  quartzites  and  calcareous  novaculites.  Grossularite 
also  is  associated  with  common  garnet  in  other  schists  and  gneisses.  It  is 
recorded  as  being  derived  from  melilite  and  gehlenite. 

Pyrope,  the  magnesium  garnet,  as  would  be  expected,  is  especially 
prevalent  in  peridotites  and  their  derivatives,  such  as  serpentine  and  talc, 
since  these  rocks  are  rich  in  magnesium.  It  also  occurs  in  some  basalts. 

Almandite,  one  of  the  most  widespread  of  the  pure  garnets,  occurs  in 
granites,  schists,  gneisses,  and  granulites,  and  thus  is  present  in  both 
feldspathic  and  feldspar-free  schists.  Almandite  is  also  known  in  certain 
andesites.  It  rarely  has  crystalline  forms. 

Spessartite  occurs  in  large  and  small  grains  in  contact  rocks,  in 
porphyritic  crystals  of  large  size  in  quartzites,  and  is  abundant  in  certain 
whetstone-schists.  With  topaz,  it  is  known  in  lithophysse  in  rhyolite. 


302  A  TREATISE  ON  METAMOKPHISM. 

Melanite  is  common  in  basic  eruptive  rocks  rich  in  alkali.  It  occurs 
especially  with  nephelite  and  leucite  in  phonolites,  leucitophyres,  nephe- 
liuitcs,  and  tephrites.  In  connection  with  contact  metamorphism  it  occurs 
with  wollastonite  and  fassaite.  It  is  also  found  in  many  serpentines. 

Uvarovite  is  at  home  in  the  serpentine*,  particularly  those  which  con- 
tain chromite.  It  is  also  found  in  the  marbles. 

Common  garnet,  ordinarily  a  molecular  mixture  of  two  or  more  of  the 
species  grossularite,  pyrope,  almandite,  and  melanite,  is  of  course  more 
abundant  than  the  pure  species.  It  occurs  in  such  rocks  as  amphibolites 
and  eclogites,  in  the  metamorphosed  diabases  and  gabbros,  in  the  pyroxenic 
rocks  and  their  derivatives,  and  in  the  schists  and  gneisses  both  of  igneous 
and  of  sedimentary  origin. 

Alterations. — The  minerals  into  which  garnets  alter  are  very  numerous, 
chlorite  (monoclinic,  sp.  gr.  2.71-2.725),  talc  (orthorhombic  or  monoclinic, 
sp.  gr.  2.75),  and  serpentine  (monoclinic;  sp.  gr.  2.575),  however,  being  the 
more  common  products.  Only  the  secondary  products  which  occur  on 
an  important  scale  in  the  rocks  will  be  discussed,  mere  mineralogical 
occurrences  and  pseudomorphs  being  ignored. 

Alterations  of  grossularite  are  not  described  in  the  standard  text-books; 
but  it  is  known  that  meionite  (tetragonal;  sp.  gr.  2.72)  and  zoisite  (saus- 
surite)  (massive;  sp.  gr.  3.-3.04)  are  sometimes  secondary  products  of  garnet, 
and  it  is  natural  to  suppose  that  these  minerals  are  derived  either  from 
grossularite  or  from  the  grossularite  molacule  of  common  garnet,  since 
grossularite  contains  the  elements  in  about  the  right  proportions  to  produce 
meionite  and  zoisite. 

Talc  and  serpentine  are  minerals  which  are  secondary  to  garnet,  and 
from  their  chemical  composition  ought  to  be  derived  from  the  pyrope  mole- 
cule, either  from  the  pure  garnet  or  from  the  pyrope  molecule  in  com- 
bination with  other  garnet  molecules.  Pyrope  is  known  to  alter  into 
chlorite.  As  chlorite  is  regarded  as  a  molecular  mixture  of  serpentine  and 
amesite  (crystallization  not  determined;  sp.  gr.  2.71),  equations  are  written 
for  its  alterations  into  amesite  and  into  average  chlorites.  Pyrope  further 
alters  into  enstatite  (orthorhombic;  sp.  gr.  3.2)  and  spinel  (isometric;  sp.  gr. 
3.8),  these  minerals  frequently  forming  kelyphite  rims  about  the  garnet. 

Almandite  is  recorded  as  altering  into  chlorite  and  into  hypersthene 
(orthorhombic;  sp.  gr.  3  45)  and  spinel,  which  minerals  form  kelyphite  rims 


ALTERATIONS  OF  MINERALS  OF  GARNET  GROUP.     303 

about  the  garnets.  It  seems  probable  that  in  such  cases  with  the  alman- 
dite  there  is  also  present  the  pyrope  molecule,  and  the  reactions  for  the 
formation  of  chlorite,  .spinel,  and  hypersthene,  after  almandite,  as  written 
include  the  pyrope  molecule.  In  the  case  of  the  Spurr  mine  chlorite, 
secondary  to  garnet,  the  species  has  been  determined  to  be  aphrosiderite" 
(massive;  sp.  gr.  2.90). 

Alterations  of  spessartite,  melanite,  and  uvarovite,  as  pure  species,  are 
not  described  in  the  standard  text-books. 

Common  garnet  most  frequently  alters  into  chlorite.  Often  also  it 
changes  into  epidote  (monoclinic;  sp.  gr.  3.38)  or  into  hornblende  (mono- 
clinic;  sp.  gr.  3.26).  The  mixture  of  almandite  and  pyrope  altering  into 
aphrosiderite,  and  into  hypersthene  and  spinel,  may  be  considered  as  alter- 
ations of  common  garnet.  Where  epidote  is  produced  it  is  probable  that 
the  molecules  from  which  it  is  derived  are  a  mixture  of  grossularite  and 
melanite.  Where  hornblende  is  produced  it  is  probable  that  the  molecules 
are  a  mixture  of  pyrope.  almandite,  and  melanite.  In  the  alterations  of 
the  common  garnets  any  of  the  iron  oxides,  magnetite  (isometric;  sp.  gr. 
5.174),  hematite  (rhombohedral;  sp.  gr.  5.225),  or  limonite  (amorphous; 
sp.  gr.  3.80),  may  be  produced. 

The  change  from  grossularite  to  meionite  may  be  written  as  follows: 

(1)  SCasA^SisOu+SCO^CatAleSisb^+SCaCOs+SSiOz+k. 

The  increase  in  volume  of  the  meionite,  calcite  (rhombohedral;  sp.  gr. 
2.7135),  and  quartz  (rhombohedral;  sp.  gr.  2.6535)  as  compared  with  the 
grossularite  is  54.62  per  cent. 

The  change  of  grossularite  to  zoisite  may  be  written  as  follows: 

(2)  3Ca8Al2SisOI2+5COJ+H.!O=2HCa2Al3Si3Oi3+5CaCOs+3SiO2+k. 

The  increase  in  volume  of  the  zoisite,  calcite,  and  quartz  as  compared 
with  the  grossularite  is  40.49  per  cent. 

The  alteration  of  pyrope  to  talc  may  be  written  in  two  ways,  depend- 
ing upon  whether  the  excess  of  magnesium  over  that  required  for  the  for- 

. _j i . 

oPurnpelly,  Raphael,  On  peeudomorpha  of  chlorite  after  garnet:  Am.  Jour.  Sci.,  3d  ser.,  vol.  10, 
1875,  pp.  1-4.  Penfield,  S.  L.,  and  Sperry,  F.  L.,  Pseudomorpha  of  garnet  from  l>ake  Superior  and 
Salida,  Colo.:  Am.  Jour.  Sci.,  3d  ser.,  vol."  32,  1886,  pp.  307-311. 


304  A  TREATISE  ON  METAMORPHISM. 

mation  of  talc  is  regarded  as  passing  into  magnesite  (rhombohedral ;  sp. 
gr.  3.06)  or  into  spinel.  The  first  reaction  is — 

(3)  4Mg3Al2Si3012+15H20+3C02=3H2Mg3Si4012+3MgC03+8Al(OH)3+k. 

The  increase  in  volume  of  the  talc,  magnesite  (rhombohedral;  sp.  gr.  3.06), 
and  gibbsite  (monoclinic;  sp.  gr.  2.35)  as  compared  with  the  pyrope  is  75.91 
per  cent.  The  second  reaction  is — 

(4)  4MgsAl2Si3O12+6H2O=3H2Mg3Si4O12+3MgAl2O4+2Al(OH)3+k. 

The  increase  in  volume  of  the  talc,  spinel,  and  gibbsite  as  compared  with 
the  pyrope  is  36.84  per  cent. 

The  change  of  pyrope  into  serpentine  is — 

(5)  Mg3Al2Si3O12+5H2O=H4Mg3Si2O9+2Al(OH),+SiO2+k. 

The  increase  in  volume  of  the  serpentine,  gibbsite,  and  quartz  as  compared 
with  the  pyrope  is  81.61  per  cent. 

If  amesite  (hexagonal  plates;  sp.  gr.  2.71)  is  produced  from  pyrope 
the  equation  is — 

(6)  Mg3Al2Si,0I2+2H20+C02=H4Mg2Al2Si09+MgC03+2Si02+k. 

The  increase  in  volume  of  the  amesite,  magnesite,  and  quartz  as  compared 
with  the  pyrope  is  62.26  per  cent. 

In  the  alteration  of  pyrope  to   chlorite,   supposing  an   intermediate 
chlorite  be  taken,  the  reaction  is — 

(7)  3Mg3Al2Si3012+8H2O^H16Mg9Al6Si5036+4SiO.,+k. 

The  increase  in  volume  of  the  chlorite  and  quartz  as  compared  with  the 
pyrope  is  56.02  per  cent.  Reactions  could  be  written  which  represent 
other  varieties  of  chlorite. 

The  change  of  pyrope  to  enstatite  and  spinel  is — 

(8)  Mg,Al2SisO12=2Mg  SiO3+MgAl2O4+SiO2+k. 

The  increase  in  volume  of  the  enstatite,  spinel,  and  quartz  as  compared 
with  the  pyrope  is  13.51  per  cent. 

The  alteration  of  almandite  and  pyrope  to  chlorite  (aphrosiderite), 


ALTERATIONS  OF  MINERALS  OF  GARNET  GROUP.     305 

supposing  the  Fe:Mg::  2:1,  about  the  proportion  shown  by  analysis  in  the 
case  of  the  Lake  Superior  chlorite  at  the  Spun-  mine,"  is — 

(9)  4Fe3Al2Si3012.2Mg3Al2Si3O12+15H2O=3H10Fe4Mg2Al4SiA5+6SiO2+k. 

The  increase  in  volume  of  the  aphrosiderite  and  quartz  as  compared  with 
the  garnet  is  50.98  per  cent. 

The  alteration  of  almandite  and  pyrope  to  hypersthene  and  spinel, 
supposing  the.Mg:  Fe ::  1 :  1  in  the  hypersthene,  is  as  follows: 

( 10)  Fe3Al2Si3O12.2Mg3Al2Si3O12=3MgFeSi2O6+3Mg  Al2O4+3SiO2+k. 

The  increase  in  volume  of  the  hypersthene,  spinel,  and  quartz,  as  compared 
with  the  garnet,  is  12.66  per  cent.  If  a  hypersthene  be  produced  which  is 
less  rich  in  iron,  the  amount  of  pyrope  molecule  in'  the  original  garnet 
must  be  increased. 

The  alteration  of  grossularite  and  melanite  to  epidote,  supposing  an 
average  epidote  be  produced,  in  which  the  Al:  Fe::  2: 1  is  probably — 

(11)  2Ca3Al2Si,012.Ca3Fe28isO1,+5CO2+H20=2HCa2Al2FeSi3Ols+5CaCO,+3SiO2+k. 

The  increase  in  volume  of  the  epidote,  calcite  (rhombohedral,  sp.  gr. 
2.7135),  and  quartz,  as  compared  with  the  garnet,  is  40.88  per  cent. 
Similar  equations  can  be  written  in  which  the  pyrope  molecule  takes  the 
place  of  the  grossularite  molecule  in  large  part,  In  this  case  magnesite, 
instead  of  calcite,  would  be  produced.  Other  reactions  could  be  written 
for  the  formation  of  epidote,  in  which  the  original  molecule  is  a  combination 
of  grossularite,  pyrope,  and  melanite.  The  simplest  case  is  as  follows: 

(12)  C%Al2Si3O12.Mg3Al2Si3012.Ca3Fe2Si3OI2+H2O+5CO2= 

2HCa2Al2FeSi,013+2CaC03+3MgCOs+3SiO2+k. 

In  this  case  the  increase  in  volume  of  the  epidote,  calcite,  magnesite,  and 
quartz,  as  compared  with  the  garnet,  is  39.53  per  cent. 

The  reaction  for  the  passage  of  pyrope,  almandite,  and  melanite  into 
hornblende  may  be  written  in  many  ways,  depending  upon  the  composition 
of  the  particular  hornblende  produced.  Taking  the  case  of  an  average 
hornblende,  in  which  there  are  five  of  the  actinolite  molecules  to  two  of  the 

«Penfield,  S.  L.,  and  Sperry,  F.  L.,  On  pseudomorphs  of  garnet  from  Lake  Superior  and 
Salida,  Colo.:  Am.  Jour.  Sci.,  3d  ser.,  Vol.  32,  1886,  pp.  307-311. 

MON    XLVII — 04 20 


306  A  TREATISE  ON  METAMORPHISM. 

aluminous  molecules,  in  which  the  MgO:  FeO::  2:1,  and  A1203:  FeuO3::  3:  1, 
the  reaction  is  as  follows: 


(  13  )     3  [2Mgs  Al2Si3O12.  FesAl2Si3O12.  Ca^SiA-,]  +4CO,  = 

5CaMg2FeSi4012.2[(MglFe2)  (  Al9Fe3)Si60S6]  +4CaCO3+4SiO2+k. 

The  increase  in  volume  of  the  hornblende,  calcite,  and  quartz,  as  compared 
with  the  garnet,  is  24.55  per  cent. 

The  alterations  of  the  ferriferous  garnets  frequently  produce  iron 
carbonate  or  iron  oxides.  No  reactions  are  written  to  illustrate  these 
changes;  nor  would  it  be  easy  to  express  these  alterations  by  reactions 
without  knowing  what  becomes  of  the  remainder  of  the  garnet  material. 

Of  course,  the  alterations  which  are  written  above,  instead  of  taking 
place  separately,  may  occur  simultaneously.  Thus  the  garnet  may  be  a 
complex  one,  which  contains  molecules  of  several  of  the  simple  garnets, 
and  there  would  be  simultaneously  produced  a  considerable  number  of 
secondary  minerals.  Thus,  chlorite  and  hornblende,  chlorite  and  epidote, 
or  epidote  and  hornblende,  might  be  simultaneously  produced.  For  defi- 
nite cases  such  as  these,  reactions  might  be  written  by  combining  the  reac- 
tions for  the  production  of  the  individual  minerals. 

An  examination  of  the  equations  as  written  shows  that  in  almost  all 
cases,  simultaneously  with  the  production  of  the  minerals  which  are  recorded 
as  secondary  to  garnet,  quartz  also  appears,  and  in  some  cases  calcium 
carbonate  also  must  separate,  which  may  be  deposited  in  the  form  of  calcite. 
Less  frequently  siderite  and  iron  oxide  form.  It  is  well  known  that  with 
the  minerals  chlorite,  epidote,  hornblende,  etc.,  secondary  to  garnet,  quartz, 
and  calcite  are  often  found,  and  that  with  serpentine,  talc,  spinel,  hyper- 
sthene,  and  enstatite,  quartz  is  often  found.  However,  the  quartz  and  calcite 
are  usually  not  regarded  as  derived  from  the  garnet  and  called  minerals 
secondary  to  them.  But  the  equations  clearly  show  that  these  minerals 
should  be  regarded  as  secondary  to  garnet,  just  as  certainly  as  epidote, 
chlorite,  etc.  The  almost  universal  presence  of  quartz  with  the  minerals 
mentioned,  and  the  frequent  presence  of  calcite,  are  thus  completely 
explained.  The  equations  also  seem  to  demand  in  the  alteration  to  serpen- 
tine and  talc  that  gibbsite  or  diaspore  shall  be  produced.  However,  some 
of  the  alumina  may  unite  with  silica  and  water  and  form  kaolin.  The 
equations  suggest  that  a  search  be  made  for  gibbsite,  diaspore,  and  kaolin 


ALTERATIONS  OF  MINERALS  OF  GARNET  GROUP.  307 

where  the  serpentines  and  talcs  are  secondary  to  garnet.  Of  course,  in 
many  cases  the  silica,  calcium  carbonate,  and  possibly  the  excess  of 
aluminum  hydrate,  may  be  dissolved  and  transported  elsewhere,  and  thus 
their  absence  would  be  no  proof  that  the  compounds  were  not  really  pro- 
duced by  the  alteration  of  the  garnet. 

The  alterations  of  the  various  kinds  of  garnet  into  different  combina- 
tions of  the  following  minerals,  serpentine,  talc,  chlorite,  epidote,  and  zoisite, 
magnesite,  and  gibbsite  (equations  2,  3,  4,  5,  6,  7,  9,  11,  12),  are  all 
alterations  of  hydration,  and  the  majority  of  them  of  carbonation  and 
desilication.  These  reactions  are  notable  in  the  amount  of  increase  in 
volume,  ranging  from  36  to  80  per  cent.  This  increase  in  volume  is  a 
natural  consequence  of  the  high  specific  gravity  of  the  garnet.  The  altera- 
tions of  grossularite  to  meionite,  calcite,  and  quartz  (equation  1),  and  of 
pyrope,  almandite,  and  melanite  to  hornblende,  calcite,  and  quartz  (equa- 
tion 13),  are  alterations  of  carbonation  and  desilication.  There  can  be  no 
better  illustrations  of  reactions  characteristic  of  the  zone  of  •katamorphism. 
It  will  be  seen  (pp.  683-685)  that  the  development  of  garnet  is  a  process 
of  the  zone  of  anamorphism  where  the  pressure  is  great  and  the  tempera- 
ture probably  high.  Naturally  the  extensive  destruction  of  garnet  is  a 
process  of  the  upper  physical-chemical  zone. 

The  alterations  of  pyrope  to  enstatite,  spinel,  and  quartz  (equation  8), 
and  of  almandite  and  pyrope  together  to  hypersthene  and  spinel  (equation 
10),  are  common  reactions.  They  do  not  involve  hydration.  They  do, 
however,  involve  desilication.  The  increase  in  volume  for  these  changes 
is  comparatively  small,  12  or  13  per  cent.  One  would  expect  that  these 
reactions  would  take  place  either  in  the  lower  part  of  the  belt  of  cementa- 
tion or  possibly  in  the  upper  part  of  the  zone  of  anamorphism. 


308  A  TREATISE  ON  METAMORPHISM. 

CHRYSOLITE    GROUP. 
FOKSTEBITE,  OLIVINE,  AM)  FATALITE. 

The  chrysolite  group  includes — 

Forsterite: 
Mg,SiO4. 
Orthorhornbic. 
Sp.  gr.  3.21-3.33. 

Olivine: 

(MgFe)2SiO4  where  Mg:Fe::16:l,  12:1,  to  2:1,  in  the  last  case  the  mineral  being 

known  as  hyalosiderite.     (Sp.  gr.  3.566.) 
Orthorhombic. 
Sp.  gr.  3.2-3.6  according  to  Hintze,  but  ordinarily  being,  according  to  Dana,  3.27-3.37. 

Fayalite: 
Fe2SiO4. 
Orthorhombic. 
Sp.  gr.  4.1. 

occurrence. — Tscheiinak  considers  olivine  as  an  isomorphous  mixture  of 
fayalite  and  forsterite.  The  occurrence  of  the  three  minerals  is  the  same, 
except  that  fayalite  and  forsterite  are  not  nearly  so  widely  known  as  the 
intermediate  common  mineral,  olivine.  Olivine  is  an  abundant  constituent 
in  intermediate  and  basic  igneous  rocks,  both  plutonic  and  volcanic,  in  lavas 
and  tuffs  alike.  In  rare  cases  in  the  volcanic  rocks  fayalite  occurs,  as,  for 
instance,  in  nodules  in  volcanic  rocks  and  in  lithophysse  of  the  rhyolites  of 
the  Yellowstone  Park.  Forsterite  also  very  rarely  occurs  in  connection 
with  volcanic  rocks.  Olivine  is  also  an  accessory  constituent  in  the  very 
basic  schists  and  gneisses,  such  as  the  amphibolites,  pyroxenites,  eclogites, 
etc.  Finally,  it  not  infrequently  occurs  in  marbles.  In  rocks  of  this  class 
forsterite  also  rarely  occurs.  It  therefore  appears  that  the  chrysolite  group 
of  minerals  occurs  most  abundantly  as  original  constituents,  but  are  also 
rather  widely  found  as  secondary  developments  in  the  metamorphosed  rocks, 
including  both  the  carbonates  and  the  basic  schists. 

Alterations. — The  alterations  of  fayalite  and  forsterite  are  exceptional; 
therefore  the  chief  alterations  which  are  considered  are  those  which  pertain 
to  olivine. 

The  most  common  alteration  of  olivine  is  to  serpentine  (monoclinic; 
sp.  gr.  2.50-2.65).  This  is  a  change  from  an  anhydrous  orthosilicate  to  a 
hydrous  orthosilicate.  Doubtless  this  explains  why  serpentine  rather  than 


ALTERATIONS  OF  OLIVINE.  309 

talc  develops  so  generally  from  the  olivines,  because  talc  is  a  metasilicate. 
Ordinarily  accompanying  the  serpentine  one  or  more  of  the  following 
minerals  may  be  found:  Trernolite  (monoclinic;  sp.  gr.  3.0),  actinolite 
(monoclinic;  sp.  gr.  3.10),  talc  (orthorhombic  or  monoclinic;  sp.  gr.  2.75), 
hydrotalcite  (hexagonal;  sp.  gr.  2.04-2.09),  magnesite  (rhombohedral;  sp.  gr. 
3.0(5),  breunnerite  (rhombohedral;  sp.  gr.  3-3.2),  siderite  (rhombohedral; 
sp.  gr.  3.83-3.88),  quartz  (rhombohedral;  sp.  gr.  2.6535),  opal  (amorphous; 
sp.  gr.  2.15),  magnetite  (isometric;  sp.  gr.  5.174),  chromite  (isometric;  sp.  gr. 
4.445),  hematite  (rhombohedral;  sp.  gr.  5.225),  and  limonite  (amorphous; 
sp.  gr.  3.80).  One  of  the  most  frequent  combinations  of  minerals  with 
serpentine  is  magnesite,  quartz  or  opal,  and  magnetite.  Frequently  the 
magnetite  may  partially  or  completely  replace  the  hematite  or  limonite. 
The  formation  of  the  serpentine  is  frequently  accompanied  by  tremolite  or 
actinolite  with  iron  oxide.  It  is  much  less  frequently  accompanied  by  talc. 
In  some  instances  the  olivine  has  passed  directly  into  magnesium  carbonate 
and  hematite  or  limonite,  but  the  former  commonly  being  largely  removed 
in  solution. 

Other  alterations  of  olivine  are  into  anthophyllite  (orthorhombic;  sp. 
gr.  3.15)  into  actinolite,  hematite,  and  spinel  (isometric;  sp.  gr.  3.8),  but 
these  are  by  no  means  comparable  in  importance  to  the  change  to  serpentine. 

Beginning  with  the  simplest  alteration  to  serpentine,  if  an  olivine  be 
taken  in  which  the  magnesium  is  to  the  iron  as  3:1,  and  magnetite  being 
the  only  mineral  which  accompanies  the  serpentine,  the  reaction  may  be 
written  as  follows: 

( 1 )  3MgaFeSi  A + 6H2O+0 =3H4Mg3Si2O9+ Fe3O,+k. 

The  increase  in  volume  of  the  serpentine  and  magnetite  as  compared  with 
the  olivine  is  29.96  per  cent. 

Supposing  the  magnesium  is  to  the  iron  as  1:1  and  the  iron  passes  into 
magnetite,  the  reaction  is— 

(2)  3MgFeS10,+2H2O+O=H4Mg3SiA+FeA+SiO2+k. 

The  increase  in  volume  of  the  serpentine,  magnetite,  and  quartz  as  compared 
with  the  olivine  is  15.19  per  cent. 

If  it  be  supposed  that  a  third  of  the  magnesium  passes  into  magnesite, 
and  that  silica  also  separates,  the  reaction  may  be  written  as  follows: 

(3) 


310  A  TREATISE  ON  METAMORPHISM. 

The  increase  in  volume  of  the  serpentine,  magnetite,  magnesite,  and  quartz 
as  compared  with  the  olivine  is  37.13  per  cent.  Supposing  the  Mg  and  Fe 
are  present  in  equal  proportions,  the  equation  stands — 

(4)  3Mg2Fe2Si208+4H20+2  O=2H4Mg3Si2O9+2FeA+2SiO2+k. 

In  this  case,  the  olivine  of  which  nearly  corresponds  to  that  of  many  rocks, 
the  increase  in  volume  is  12.43  per  cent. 

It  would  be  easy  to  write  other  equations  for  different  proportions  of 
maglaesium  and  iron  in  the  olivine,  but  this  seems  unnecessary.  Also  it 
would  be  easy  to  write  reactions  by  which  other  forms  of  iron  compounds 
than  magnetite  are  produced,  such  as  siderite,  hematite,  and  limonite.  If 
this  be  done,  and  the  volume  reaction  calculated,  it  will  be  found  that  the 
increase  in  volume  is  still  greater  than  when  magnetite  forms. 

Olivine  is  described  by  Becke  as  passing  into  anthophyllite  (where 
Mg :  Fe  : :  4 : 1,  3:1,  etc.,  orthorhombic ;  sp.  gr.  3.1-3.2).  If  the  proportion 
of  the  magnesium  to  the  iron  be  taken  as  3:1  in  both  the  olivine  and  the 
anthophyllite,  the  reaction  may  be  written  as  follows: 

(5)  Mg,FeSi2O8+2Si02=MgsFeSi4O12-  k. 

The  decrease  in  volume  of  the  anthophyllite  as  compared  with  the  original 
olivine  and  quartz  is  1.48  per  cent. 

Various  authors  have  also  described  the  alteration  of  olivine  into 
actinolite.  Supposing  that  the  magnesium  is  to  the  iron  as  3 : 1  in  both  the 
olivine  and  the  actinolite,  and  supposing  the  calcium  to  be  derived  from 
carbonate  and  the  silica  from  quartz,  the  reaction  is  as  follows: 

(6)  3MgsFeSi208+4CaCOs+10Si02=Mg9Fe8Ca4Si,6048+4C02-k. 

The  decrease  in  volume  of  the  actinolite  as  compared  with  the  olivine, 
calcite,  and  quartz,  is  13.34  per  cent, 

In  some  instances  the  altei-ation  into  actinolite  is  described  as  taking 
place  in  connection  with  feldspar  as  a  reaction  rim.  In  this  case  the  calcuim 
may  be  supposed  to  be  derived  from  anorthite,  as  calcium  silicate.  The 
aluminum  may  be  supposed  to  pass  into  common  spinel  and  hercynite 
(isometric;  sp.  gr.  3.93),  which  are  well  known  to  be  alteration  products  of 
olivine.  The  reaction  mav  be: 

(7)  4Mg,FeSi2O8-4CaAl2Si208=Mg8Fe,Ca,Sil6O48+3MgAlJOt+FeAlaO4--k. 


SCAPOLITE  GROUP.  311 

The  volume  decrease  of  the  actinolite  and  spinels  as  compared  with  the 
olivine  and  feldspar  is  7.  18  per  cent. 

The  reactions  in  the  alterations  of  olivine  into  tremolite  are  parallel 
with  those  for  actinolite,  with  the  exception  that  no  iron  is  present,  and  the 
mineral  therefore  probably  forms  from  forsterite.  The  reaction  may  be 
written: 

(8)     3Mg2SiO,+2CaCO3+5SiO2=2Mg3CaSi1O12+2CO,!-k. 

The  decrease  in  volume  of  the  tremolite  as  compared  with  the  forsterite, 
calcite,  and  silica  is  12.29  per  cent. 

The  alteration  of  olivine  to  serpentine  and  the  accompanying  minerals 
is  the  common  one.  It  takes  place  in  the  zone  of  katamorphism  on  a  great 
scale,  both  in  the  belt  of  weathering  and  in  the  belt  of  cementation.  Corre- 
sponding with  the  position  in  the  upper  physical-chemical  zone,  the  reactions 
occur  with  hydration,  oxidation,  expansion  of  volume,  and  liberation  of  heat. 

The  developments  of  anthophyllite,  actinolite,  and  tremolite  from 
olivine  and  actinolite,  and  of  spinel  from  olivine  and  feldspar,  are  all  deep- 
seated  reactions  of  the  zone  of  anamorphism.  Corresponding  to  this  position 
the  change  to  anthophyllite,  equation  (5),  is  a  reaction  of  silication;  the 
changes  to  actinolite  and  to  tremolite,  equations  (6)  and  (8),  silication 
and  decarbonation  ;  and  the  change  of  olivine  and  anorthite  to  actinolite 
and  spinel,  equation  (7),  rearrangement  of  the  silicates  into  denser  silicates; 
and  all  take  place  with  diminution  of  volume  and  absorption  of  heat. 

SCAPOLITE    GROUP. 
MEIONITE,  WERN'EBITE,  «nd  il\KI  V1.ITK. 

The  scapolite  group  includes: 

Meionite: 

Ca4Al6Si6O25. 

Tetragonal. 

Sp.  gr.  2.70-2.74. 

Wernerite: 


Tetragonal. 

Sp.  gr.  2.66-2.73. 

Marialite  : 


Tetragonal. 
Sp.  gr.  2.566. 


312  A  TREATISE  ON  METAMORPH1SM. 

As  is  well  known,  the  scapolite  group  is  analogous  to  the  plagioclase 
group,  both  consisting  of  sodium-aluminum-silicate  molecules  and  calcium- 
aluminum-silicate  molecules  in  various  proportions.  Wernerite  is  a 
combination  of  the  marialite  and  meionite  molecules  in  various  ratios. 
Generally  the  ratios  vary  between  2  : 1  to  1  :  3. 

occurrence. — Dana  summarizes  the  occurrence  of  the  scapolites  as  follows: 
"(1)  in  volcanic  rocks,  as  in  ejected  masses  on  Mte.  Somma  (meionite); 
(2)  in  crystalline  limestone,  often  as  the  direct  result  of  contact  meta- 
morphism;  (3)  crystalline  schists,  augite-gneiss,  etc.;  (4)  as  an  alteration 
product  of  a  plagioclase  feldspar,  sometimes  on  an  extensive  scale,  as  with 
amphibole."  ° 

Alterations. — Dana  states  that  the  scapolites  are  readily  alterable.  The 
more  common  products  of  alteration  are  kaolin  (monoclinic;  sp.  gr.  2.6-2.63), 
talc  (orthorhombic  or  monoclinic;  sp.  gr.  2.7—2.8),  muscovite  (hydromusco- 
vite,  pinite)  (monoclinic;  sp.  gr.  2.76-3.0),  and  epidote  (the  Al  and  Fe 
varying  from  (J:l  to  3:2;  monoclinic;  sp.  gr.  3.25-3.50).  It  is  also  recorded 
that  the  scapolites  alter  into  biotite  (monoclinic;  sp.  gr.  2.7-3.1),  Accom- 
panying various  of  these  alteration  products  quartz  (rhombohedral ;  sp.  gr 
2.653-2.654)  separates.  Also,  it  is  probable  that  in  connection  with  some 
of  them,  gibbsite  (monoclinic;  sp.  gr  2.3-2.4)  or  diaspore  (orthorhombic; 
sp.  gr.  3.3-3.5)  forms,  and  very  likely  also  calcite  (rhombohedral;  sp.  gr. 
2.713-2.714). 

In  writing  out  equations  for  the  alterations  to  the  above  minerals,  one 
is  handicapped  by  lack  of  knowledge  as  to  whether  the  marialite  or  the 
meionite,  or  a  combination  of  the  two,  produces  a  given  mineral.  In  this 
state  of  affairs  the  particular  molecule  is  chosen  which  is  most  analogous  to 
the  compound  produced.  It  seems  probable  that  kaolin  and  talc  together 
are  produced  from  marialite,  according  to  the  reaction : 

(1)  2Na4Al3Si9O2»Cl+9MgCO3+9H2O= 

3H4Al2SiA^3H2Mg3Si4O124-3Na,COs+2NaCl+6CO2+k. 

The  increase  of  volume  of  the  kaolin   and  talc,   as  compared  with  the 
marialite,  is  7.69  per  cent 

It  may  be  that  kaolin  and  calcite  are  also  produced  from  meionite,  as 
follows : 

(2)  Ca4Al6Si6O25+6H20+4CO,=3H4Al2Si2O9+4CaCOs+k. 

"  Dana,  J.  I).,  A  system  of  mineralogy;  Descriptive  mineralogy,  by  E.  S.  Dana,  Wiley  &  Sons,  New 
York,  6th  ed.,  1892,  p.  467. 


ALTERATIONS  OF  MEIONITE  AND  MAKIALITE.  313 

Supposing  all  of  the  CaCO3  to  remain  as  calcite,  the  increase  of  volume  is 
35.40  per  cent. 

The  passage  of  the  scapolites  into  muscovite  may  be  written  as  follows: 

For  marialite: 

(3)  2Na(Al3SiA4Cl+K2C08+2H20+2C02=2KH2Al3Si30I2+12SiOJ+2NaCH-3Na2C08+k. 

In  this  reaction,  as  in  the  case  of  the  passage  of  the  acid  feldspars  into 
muscovite,  a  large  amount  of  the  silica  separates.  The  decrease  in  volume 
of  the  muscovite  and  quartz  as  compared  with  the  marialite  is  16.74  per 
cent,  but  if  the  soluble  sodium  salts  be  also  taken  into  account  the  volume 
is  increased. 

For  meionite  the  reaction  may  be — 

(4)  Ca4Al6Si6025+K2CO3+3CO2  -r2H2O=2KH2Al3Si3O12+4CaCO3+k. 

The  increase  in  volume  of  the  muscovite  and  calcite  as  compared  with  the 
meionite  is  29.42  per  cent. 

As  the  composition  of  epidote  is  very  analogous  to  meionite,  and  as  it 
is  a  calcium-bearing  compound,  it  is  thought  likely,  where  epidote  is  second- 
ary to  a  scapolite,  that  it  is  derived  from  a  meionite  molecule.  Therefore, 
supposing  that  the  epidote  is  one  in  which  the  aluminum  is  to  the  iron  as 
2:1,  and  supposing  that  the  iron  is  derived  from  feme  oxide  (Fe203),  the 
reaction  may  be  written  as  follows: 

(5)  Ca4Al6SiA5+Fe203+4H20=2HCa2Al,FeSi30I3+2Al(OH)3+k. 

Supposing  the  hematite  (hexagonal -rhombohedral;  sp.  gr.  5.225)  to  have 
been  present  as  a  solid,  and  the  gibbsite  to  remain  as  a  solid,  the  decrease 
in  volume  is  1.62  per  cent.  It  is  thought  likely  that  iron  for  the  reaction 
is  often  derived  from  iron  carbonate  in  solution,,  combined  with  simulta- 
neous oxidation.  In  this  case  the  reaction  would  be — 

(6)  Ca1Al6Si6O25+2FeCOs+4H2O+O=2HCa,Al2FeSisO,s+2Al(OH)3+2CO2+k. 

The  increase  in  volume  of  the  epidote  and  gibbsite  as  compared  with  the 
meionite  is  7.55  per  cent. 

The  passage  of  marialite  into  kaolinite  and  talc  involves  hydratiou, 
expansion  of  volume,  and  liberation  of  heat.  The  change  of  meionite  to 
kaolinite  involves  hydration,  carbonation,  increase  in  volume,  and  libera- 
tion of  heat.  The  change  of  the  scapolites  to  muscovite  and  accompany- 
ing compounds  are  reactions  of  hydration,  carbonation,  increase  of  volume, 


314  A  TREATISE  ON  METAMORPHISM. 

and  liberation  of  heat.     The  change  of  meionite  to  epidote  is  a  reaction  of 
hydration  and  possibly  of  oxidation. 

Corresponding  with  these  facts  the  alterations  to  kaolin  and  talc  are 
known  to  take  place  in  the  zone  of  kataraorphism,  and  the  same  is  probably 
true  of  the  alterations  to  muscovite  and  epidote,  although  the  latter  reac- 
tions may  be  more  characteristic  of  the  belt  of  cementation  than  of  the  belt 
of  weathering. 

MELILITE. 

Melilite: 

(CaMgNa,),(  Aire^SiA,.     (Groth.  ) 

Tetragonal. 

Sp.  gr.  2.9-3.10. 

occurrence.  —  Melilite  has  a  widespread  distribution  in  the  leucite  and 
nephelite  rocks.  Aside  from  leucite  and  nephelite  the  most  characteristic 
associates  are  augite  and  perovskite.  Some  of  the  rocks  in  which  melilite 
occurs  are  leucitophyre,  nepheline-syenite,  and  basalt. 

Alterations.  —  The  alterations  of  this  mineral  are  not  recorded,  although 
from  its  composition  there  can  be  no  doubt  that  in  the  upper  physical- 
chemical  zone  it  decomposes  into  less  complicated  silicates. 

GEHLENITE. 

Gehlenite: 

Ca,Al2Si2O10. 
Tetragonal. 
Sp.  gr.  2.9-3.07. 

occurrence.  —  The  only  occurrence  of  gehlenite  recorded  in  rocks  is  as  a 
contact  product  in  limestone. 

Alterations.  —  According  to  Dana  it  alters  to  talc  (orthorhombic  or  mono- 
clinic;  sp.  gr.  2.75),  to  fassaite  (monoclinic;  sp.  gr.  2.965-3.291),  and  to 
grossularite  (isometric;  sp.  gr.  3.605). 

The  change  to  grossularite  involves  the  addition  of  SiO2,  thus: 


The  decrease  in  volume  of  the  grossularite  as  compared  with  the  gehlenite 
is  4.42  per  cent.  If  the  SiO2  be  added  as  a  solid,  the  decrease  in  volume 
is  18.56. 

As  gehlenite  is  so  rare,  and  the  manner  of  the  alteration  into  talc  and 
fassaite  is  not  clear,  no  attempt  is  made  to  write  equations  for  the  changes. 


VESUVIANITE  AND  ZIRCON.  315 

VESUVIANITE. 

Vesuvianite: 

HR"6Al,Si6O2i  (Clarke). 

Tetragonal. 

Sp.  gr.  3.35-3.45. 

Clarke  states  that  the  R6  in  the  typical  mineral  is  replaced  by  calcium 
and  magnesium  in  the  proportion  of  5:  1,  giving  HCa5MgAl3Si502i. 

occurrence. — Vesuvianite  occurs  in  ancient  ejections  of  Vesuvius.  It  is 
most  abundant  in  marbles.  It  is  also  found  in  various  gneisses  and  schists, 
especially  those  which  are  calcareous.  It  often  forms  in  connection  with 
contact  action.  It  is  frequently  associated  with  such  other  metamorphic 
minerals  as  garnet,  and  also  the  micas  and  chlorites. 

Alterations. — From  the  literature  it  is  impracticable  to  ascertain  which 
particular  garnet,  mica,  or  chlorite  forms  from  a  certain  Vesuvianite,  and 
the  accompanying  minerals  which  must  simultaneously  form  are  unknown: 
it  therefore  does  not  seem  advisable  to  attempt  to  write  equations  represent- 
ing the  alterations,  since  they  must  be  so  largely  speculative. 

ZIRCON    GROUP. 

The  only  important  rock-making  mineral  of  the  zircon  group  is  zircon. 

Zircon: 

ZrSiO,. 
Tetragonal. 
Sp.gr.  4.66-4.70. 

occurrence. — Zircon  is  especially  common  in  marble.  It  also  occurs  both 
in  massive  igneous  rocks,  such  as  syenite  and  granite,  and  in  the  schists  and 
gneisses. 

Alterations. — According  to  Clarke  the  only  alteration  described  is  that  of 
hydration,  producing  hydrous  zircon  (malacoii)  (tetragonal;  sp.  gr.  3.905), 
the  reaction  being: 

3Zr8iO4+  H2G-H2Zr,,SisO1.'.- 

The  increase  in  volume  in  the  change  is  24.05  per  cent. 


316  A  TREATISE  ON  METAMORPH1SM. 

ALUMINUM-SILICATE    GROUP. 
TOPAZ,  ANDALl'Sm:,  SILLIMAMTE,  AJiD  CYAXITE. 

The  aluminum-silicate  group  includes — 

Topaz: 

AlaF2SiO4  or  Al2(F,OH)2SiO4. 

Orthorhombic. 

Sp.  gr.  3.4-3.6. 

Andalusite: 
AlsSiO5. 
Orthorhombic. 
Sp.  gr.  3.16-3.20. 

Sillimaii  !li: 
Al2SiO5. 
Orthorhoniliic. 
Sp.  gr.  3.23-b.24. 

Cycmite  (disthene): 
Al2Si05. 
Triclinic. 
Sp.  gr.  3.56-3.67. 

occurrence. — Topaz  is  a  much  less  common  mineral  than  andalusite, 
sillimanite,  and  cyanite.  Like  them,  it  occurs  in  the  schists  and  gneisses 
of  sedimentary  origin,  especially  those  in  which  other  fluorine  minerals  are 
found,  such  as  tourmaline  and  beryl.  Unlike  andalusite,  sillimanite,  and 
cyanite,  it  is  sometimes  found  in  cavities  in  fresh  volcanic  rocks,  as,  for 
instance,  rhyolite. 

Andalusite  is  a  frequent  constituent  of  the  metamorphosed  sedimentary 
rocks,  especially  of  the  argillaceous  kinds.  It  often  occurs  in  crystals, 
including  many  other  minerals  in  the  partly  metamorphosed  sedimentary 
rocks;  but  is  also  found  in  large,  well-formed  crystals  in  the  schists. 
Frequently  in  the  metamorphosed  sedimentary  rocks  its  development  has 
been  promoted  by  the  contact  effect  of  igneous  rocks,  especially  the 
granitic  rocks.  Its  most  characteristic  associates  are  sillimanite  and  cyanite. 
With  the  former  it  frequently  has  parallel  intergrowths.  Also  it  is  fre- 
quently associated  with  garnet  and  staurolite,  and  not  infrequently  with 
tourmaline.  Andalusite  is  rare,  if  indeed  not  altogether  absent  in  the 
metamorphosed  igneous  rocks. 

Sillimanite  is  a  common  mineral  in  the  strongly  metamorphosed  sedi- 
metary  rocks,  such  as  schists  and  gneisses,  where  it  frequently  replaces 


ALUMINUM-SILICATE  GROUP.  317 

andalusite  to  a  large  extent.  Like  andalusite,  its  development  may  be 
promoted  by  the  presence  of  intrusive  rocks,  especially  granites.  In  such 
cases  sillimanite  frequently  develops  nearer  the  intrusive  masses  than  does 
the  andalusite,  the  sillimanite  therefore  being  the  mineral  which  forms  under 
conditions  of  more  advanced  metamorphism.  It  is  frequently  associated 
with  garnet  and  with  spinel  and  staurolite,  sometimes  with  iolite  (cordierito). 
Sillimanite  is  derived  from  andalusite,  biotite,  corundum,  cyanite,  diaspore, 
and  gibbsite. 

The  occurrence  and  associates  of  cyanite  are  similar  to  those  of  silli- 
manite; but  a  very  frequent  additional  associate  is  corundum,  and  where 
formed  by  the  assistance  of  igneous  rocks  the  cyanite  is  likely,  on  the 
average,  to  be  closer  to  the  intrusive  than  the  sillimanite,  although  of 
course  they  ordinarily  overlap.  As  a  metamorphic  mineral,  cyanite  is 
derived  from  andalusite,  corundum,  diaspore,  and  gjbbsite. 

Tremolite,  actinolite,  and  diopside  are  frequent  associates  of  andalusite, 
sillimanite,  and  cyauite,  especially  of  the  last  two. 

The  special  homes  of  the  aluminum-silicate  group  of  minerals  are 
the  metamorphosed  argillaceous  sedimentary  rocks.  As  is  well  known, 
kaolin  is  one  of  the  chief  constituents  of  such  rocks,  and  doubtless  it  is 
from  this  mineral  in  large  part,  under  deep-seated  conditions,  that  the 
aluminum-silicate  minerals  are  formed.  If  it  be  supposed  that  these  heavy 
minerals  develop  from  kaolin,  the  process  would  be  one  of  dehydration 
and  separation  of  silica.  This  silica  may  separate  either  as  quartz  or  may 
unite  with  other  compounds,  such  as  calcium  and  magnesium  or  other 
bases,  to  form  silicates.  The  breaking  up  of  the  kaolin  may  be  repre- 
sented by  the  following  equation: 


(1)  H^AljSijO^A^Si 

Supposing  the  mineral  produced  were  andalusite,  the  volume  of  the  anda- 
lusite and  quartz  is  25.40  per  cent  less  than  that  of  the  kaolin.  If  it  be 
supposed  that  calcium  carbonate  is  present  at  the  same  time,  and  that  the 
freed  silica  unites  with  it,  the  equation  may  be  written  : 

(2)  H4Al2Si2O,+CaCOs=Al8SiO5+CaSiO3+2H2O+CO2+k. 

In  this  case  the  volume  of  the  andalusite  and  wollastonite  is  32.32  per  cent 
less  than  that  of  the  kaolin  and  calcite.  If  the  heavier  mineral  silli- 
manite or  cyanite  be  produced  the  decrease  in  volume  is  even  greater. 


318  A  TREATISE  ON  METAMORPHISM. 

While  for  the  sake  of  simplicity  wollastonite  is  supposed  to  form,  the 
more  frequent  association  of  the  aluminum-silicate  group  is  with  tremolite, 
actinolite,  and  diopside.  For  the  first  and  last  of  these  minerals  the  freed 
silica  unites  with  the  calcium  and  magnesium  together,  and  for  the  second 
with  the  calcium,  magnesium,  and  iron.  The  equations  representing  the 
changes  are  analogous  to  (2),  and  the  volume  changes  are  in  the  same 
direction. 

Alterations. — The  standard  stated  alterations  of  the  aluminum-silicate 
group  are  to  talc  (steatite)  (massive;  sp.  gr.  2.75)  and  to  muscovite 
(damourite)  (monoclinic;  sp.  gr.  2.88).  It  is  recorded  also  that  topaz  and 
,-nidalusite  alter  to  kaolin  (monoclinic;  sp.  gr.  2.615).  Occasionally  also 
andalusite  may  alter  into  the  heavier  mineral  cyanite  (triclinic;  sp.  gr. 
3.56-3.67). 

The  alterations  of  the  minerals  into  talc  require  an  entire  change  of 
base;  that  is,  from  aluminum  silicates  to  magnesium  silicates.  The  reac- 
tions being  those  of  the  zone  of  katamorphism,  the  most  probable  source 
of  the  magnesium  is  doubtless  the  carbonate,  which  may  be  derived  from 
the  decomposition  of  magnesium  rocks  such  as  the  pyroxenites,  olivinites, 
etc.  The  process,  however,  requires  the  separation  of  aluminum  either  as 
corundum  (rhombohedral ;  sp.  gr.  4.025),  coruudophilite  (monoclinic;  sp. 
gr.  2.90),  diaspore  (orthorhombic ;  sp.  gr.  3.40),  gibbsite  (mouoclinic;  sp. 
gr.  2.35),  or  some  other  form.  Since  the  reaction  takes  place  in  the  upper 
physical-chemical  zone,  gibbsite  will  be  regarded  as  the  product  formed. 
The  change  of  the  aluminum-silicate  minerals  to  muscovite  requires  the 
addition  of  potassium.  This  is  doubtless  derived  from  the  liberation  of 
potassium  during  the  decomposition  of  the  potash  feldspars,  and  will  there- 
fore be  regarded  as  added  as  a  carbonate.  The  change  from  audalusite  to 
cyanite  is  simply  a  molecular  one,  the  result  being  a  mineral  of  great 
specific  gravity.  It  has  already  been  seen  that  andalusite  is  a  product  of 
less  intense  metamorphism,  and  that  more  intense  rnetamorphism  produces 
sillimanite  and  cyanite.  The  change  of  andalusite  to  these  heavier  minerals 
is  therefore  one  which  requires  deep-seated  conditions,  and  is  characteristic 
of  the  zone  of  katamorphism. 

The  equations  representing  the  change  of  andalusite,  sillimanite,  and 
cyanite  to  talc  with  gibbsite  may  be  written  as  follows: 

(1)     4Al,SiO6+3MgCO8+13H2O=H2Mg,Si4O12+SAl(OH)3+3CO2+k. 


ALUMINUM-SILICATE  GROUP..  319 

The  decrease  in  volume  of  the  talc  as  compared  with  the  andalusite  is  32.37 
•per  cent;  as  compared  with  the  sillimanite,  31.20  per  cent;  as  compared 
with  the  cyanite,  23.12  per  cent.  But  if  the  gibbsite  be  included  as  a  solid 
the  increases  in  volume  are  97.67  per  cent,  101.09  per  cent,  and  124.71  per 
cent,  respectively. 

The  change  of  the  three  minerals  to  kaolin  may  be  written  as  follows: 

(2)  2Al2SiO6+5H2O=H4Al2SiA+2Al(OH)3+k. 

The  change  in  volume  of  the  kaolin  as  compared  with  the  andalusite  is  a 
decrease  of  3.15  per  cent;  as  compared  with,  the  sillimanite,  a  decrease  of 
1.47  per  cent;  and  as  compared  with  the  cyanite,  an  increase  of  10.11  per 
cent.  But  if  the  gibbsite  be  a  solid,  the  increases  in  volume  are  61.87 
per  cent,  64.67  per  cent,  and  84.02  per  cent,  respectively. 

The  alterations  of  the  same  minerals  to  muscovite  (damourite)  may  be 
written  as  follows: 

(3)  6Al2Si05+K2CO3+llH2O=2H2KAlsSi3O12+6Al(OH)8  +  C02+k. 

The  decrease  in  volume  of  the  muscovite  as  compared  with  the  andalusite 
is  9.55  per  cent;  as  compared  with  the  sillimanite,  7.98  per  cent;  the 
increase  as  compared  with  the  cyanite  is  2.83  per  cent.  But  if  the  gibbsite 
be  regarded  as  a  solid,  the  increases  in  volume  are  55.47  per  cent,  58.16 
per  cent,  and  76.74  per  cent,  respectively. 

The  alterations  of  the  aluminum-silicate  minerals  to  talc,  kaolin,  or 
muscovite,  with  the  accompanying  gibbsite,  are  all  reactions  of  hydratiou. 
They  involve  great  increase  of  volume,  from  55  to  125  per  cent.  To 
produce  the  original  heavy  aluminum-silicate  minerals  in  the  zone  of 
anamorphism  undoubtedly  required  great  condensation  of  volume.  When 
the  reactions  are  reversed  in  the  zone  of  katamorphism,  there  is  a  corre- 
spondingly great  expansion  of  volume.  The  change  of  the  heavy  aluminum- 
silicate  minerals  to  the  much  lighter  hydrous  minerals  gives  one  of  the  best 
illustrative  cases  of  typical  reactions  of  the  zone  of  katamorphism. 

The  change  of  andalusite  to  cyanite,  as  already  explained,  being  a 
molecular  one,  involves  a  volume  relation  inversely  as  the  specific  gravity, 
and  therefore  by  the  change  the  volume  is  decreased  12.03  per  cent.  The 
change  of  andalusite  to  cyanite  is  a  reaction  of  the  zone  of  anamorphism. 


320  A  TREATISE  ON  METAMORPH1SM. 

EPIDOTE    GROUP. 
ZOISITE,  EPIDOTE,  PIEDMOJiTTTE,  AND  ALLANITE. 

The  epidote  group  includes  the  following  minerals: 

Zoisite: 

C%(AlOH)Al.2(SiO4),. 
Orthorhombic. 
Sp.  gr.  3.25-3.37. 

JUpidote: 

Ca,(AlOH)(AlFe),(8iO4),  where  Al:Fe  as  6:1  to  3:2. 

Monoclinic. 

Sp.  gr.  3.25-3.50. 

Piedmontite: 

Ca,  ( A10H)  ( Mn  Al ) .,  ( SiO4) ,. 

Monoclinic. 

Sp.  gr.  3.404. 

Allanite  (orthite): 

Ca,(AlOH)  (AlCeFe)a(SiO4)s. 

Monoclinic. 

Sp.  gr.  3.5-4.2. 

occurrence. — Zoisite  is  not  known  as  an  original  pyrogenic  constituent  of 
igneous  rocks.  It  is  found  in  the  schists  and  gneisses,  especially  those 
containing  the  amphiboles.  Thus  it  is  very  common  in  the  amphibolites, 
glaucophane-schists,  eclogites,  etc.  Zoisite  frequently  occurs  with  albite 
as  one  of  the  constituents  of  the  so-called  saussurites,  which  develop  as 
an  alteration  of  the  basic  feldspars,  especially  in  gabbros.  Zoisite  also 
occurs  in  the  altered  granites  and  other  acid  igneous  rocks,  although  it  is, 
on  the  whole,  less  abundant  than  in  the  more  basic  rocks,  but  in  some 
localities  it  is  plentiful  even  in  the  acid  rocks.  Zoisite  is  a  very  frequent 
constituent  in  grits,  graywackes,  and  other  sediments  of  similar  composition. 
In  such  rocks  the  minerals  were  partly  altered  to  zoisite  during  the  forma- 
tion of  the  sedimentary  rocks,  and  this  zoisite  is  to  be  classed  with  the 
allogeiiic  constituents  of  the  mechanical  sediments.  Zoisite  further  develops 
in  the  altered  sedimentary  rocks  as  a  frequent  and  sometimes  abundant 
product  of  metamorphism.  From  the  foregoing  statement  of  occurrence  it 
is  plain  that  zoisite  develops  in  the  zone  of  katamorphism,  and  especially 
in  the  belt  of  cementation.  As  shown  under  the  discussion  of  the  other 
minerals,  it  is  seen  that  zoisite  may  be  derived  from  the  folio  wing  minerals: 
Corundum,  diaspore,  gibbsite,  grossularite,  and  the  plagioclases. 


OCCURRENCE  OF  EPIDOTE.  321 

Epidote,  like  zoisite,  is  rarely  if  ever  a  pyrogenic  constituent  in  igneous 
rocks.  It  is,  however,  a  secondary  constituent  in  all  varieties  of  metamor- 
phosed igneous  rocks,  whether  plutonic  or  volcanic,  whether  lavas  or  tuffs. 
It  is  an  allogenic  constituent  of  the  sedimentary  rocks,  and  it  extensively 
develops  in  the  sedimentary  rocks  as  a  secondary  product.  It  is  particularly 
likely  to  form  in  rocks  rich  in  calcium  and  iron,  whether  igneous  or  sedi- 
mentary; and  thus  is  especially  abundant  in  those  metamorphosed  igneous 
rocks  which  contain  ferriferous  varieties  of  pyroxene  and  amphibole,  and 
in  metamorphic  sedimentary  rocks  which  contain  a  considerable  amount 
of  calcium,  as,  for  instance,  calcareous  schists  and  gneisses  and  marble. 
In  the  metamorphosed  rocks  epidote  occurs  alike  in  those  which  have  a 
strongly  developed  schistose  or  gneissose  structure  and  in  those  which  have 
merely  undergone  metasomatic  change.  It  is  found  as  one  of  the  important 
filling  constituents  of  amygdaloids.  It  frequently  develops  at  the  contact 
of  two  rocks,  especially  an  igneous  rock  with  other  rocks,  either  igneous  or 
sedimentary.  A  list  of  different  rock  species  which  contain  epidote  includes 
almost  every  variety  of  massive,  schistose,  semischistose,  and  little  altered 
igneous  and  sedimentary  rocks.  Epidote  is,  in  fact,  one  of  the  most 
important  secondary  constituents  of  all  the  silicates.  It  is  an  almost 
constant  accompaniment  of  the  chlorites.  Wherever  the  calcium-iron- 
magnesium-silicate  rocks  break  up,  the  magnesium  passing  into  chlorite,  a 
part  of  the  calcium  and  iron  is  likely  to  pass  into  epidote.  The  equations 
for  these  alterations  may  be  found  under  the  minerals  from  which  epidote 
is  derived.  Where  epidote  becomes  so  abundant  as  to  be  a  chief  constituent 
it  may  give  a  name  to  a  rock;  for  instance,  epidosite.  From  the  foregoing 
statements  it  is  apparent  that  epidote  develops  abundantly  under  mass- 
static  and  under  mass-mechanical  conditions.  It  forms  with  ease  and  on  a 
great  scale  in  the  belt  of  cementation  of  the  zone  of  katamorphism,  and  it 
is  probable  that  it  develops  to  some  extent  in  the  zone  of  anamorphism. 
Whether  it  forms  at  all  in  the  belt  of  weathering  can  not  be  stated. 
Epidote  is  derived  from  the  following  minerals:  Anorthoclase,  augite, 
biotite,  garnet,  hornblende,  melanite,  microcliue,  orthoclase,  the  plagio- 
clases,  and  the  scapolites. 

Piedmontite,  or  manganese-epidote,  is  apt  to  replace  epidote  in  those 
schists  and  gneisses  in  which  manganese  happens  to  be  an  important  con- 
stituent. Thus  it  is  rather  common  in  certain  manganese-bearing  schists  of 

MON   XLVII- 


322  A  TREATISE  ON  METAMORPHISM. 

Japan,  in  the  manganese-chlorite-sericite-gneisses  of  eastern  United  States, 
and  at  other  localities.  In  some  cases  piedmontite  occurs  as  nuclei  sur- 
rounded by  ordinary  epidote.  Piedmontite  is  occasionally  so  abundant  as 
to  be  one  of  the  chief  constituents  of  rocks. 

Allanite  occurs  as  an  original  subordinate  constituent  of  a  great  number 
of  eruptive  rocks,  such  as  granite,  rhyolite,  diorite,  tonalite,  andesite,  dacite, 
and  syenite.  In  short,  it  is  a  common  accessory  in  the  acid  and  intermediate 
eruptives,  but  is  not  so  characteristic  of  the  basic  eruptive  rocks.  It  also 
occurs  in  the  metamorphic  rocks,  such  as  the  schists  and  gneisses,  especially 
those  which  are  calcareous,  and  it  may  occur  also  in  the  marbles. 

Alterations. —  Definite  alterations  of  zoisite,  epidote,  and  piedmontite  are 
not  recorded.  But  it  is  certain  in  the  belt  of  weathering  that  zoisite  and 
epidote  break  up  into  calcite  (rhombohedral ;  sp.  gr.  2.7135),  quartz  (rhom- 
bohedral;  sp.  gr.  2.6535),  iron  oxides,  kaolin  (monoclinic;  sp.  gr.  2.615), 
and  perhaps  gibbsite  (monoclinic;  sp.  gr.  2.35);  and  piedmontite  and 
allanite  alter  into  other  minerals  in  a  similar  fashion. 

It  has  already  been  seen  that  in  the  alteration  of  mica,  pyroxene, 
amphibole,  and  other  minerals  chlorite  and  zoisite  are  frequent  simultaneous 
products  which  together  use  up  all  the  material  of  the  original  minerals. 
It  has  also  been  noted  that  the  chlorite  and  epidote  are  abundantly 
developed  together  in  the  sedimentary  rocks.  If  the  conditions  so  change 
that  these  sedimentary  rocks  or  other  rocks  in  which  epidote  and  zoisite 
have  formed  in  the  zone  of  katamorphism  become  so  deeply  buried  as  to 
pass  into  the  zone  of  anamorphism,  it  is  highly  probable  that  the  consti- 
tuents which  form  epidote  and  zoisite  and  those  which  form  chlorite  reunite 
to  produce  minerals  that  are  on  the  average  denser,  such  as  mica,  amphibole, 
pyroxene,  etc.,  out  of  which  they  are  originally  developed.  This  is  believed 
to  be  probable  from  the  fact  that  in  the  most  profoundly  metamorphosed 
sedimentary  rocks,  those  which  are  true  schists  and  gneisses,  little  or  no 
epidote  and  chlorite  is  contained,  unless  they  have  again  been  subjected  to  the 
conditions  of  the  upper  physical-chemical  zone.  Such  schists  and  gneisses, 
having  been  derived  from  and  traced  into  ordinary  sediments,  in  all  prob- 
ability did  originally  contain  both  chlorite  and  epidote,  which  have  doubt- 
less united  to  reproduce  heavy  minerals  similar  to  those  from  which  epidote 
and  chlorite  formed  originally. 


ALTERATIONS  OF  ZOISITE  AND  EPIDOTE.  323 

It  is  not  easy  to  approach  accuracy  in  writing  equations  for  the  altera- 
tions of  the  epidotes  in  the  belt  of  weathering.  In  the  equations  given 
below  it  is  supposed  that  the  calcium  passes  into  carbonate,  that  the 
Al  (OH)  goes  into  gibbsite,  that  the  remainder  of  the  aluminum  goes  into 
kaolin,  and  that  the  excess  of  silica  separates  as  quartz.  In  the  epidote  the 
Al  is  supposed  to  be  to  the  Fe  as  2:1,  and  the  iron  is  supposed  to  pass  into 
limonite  (amorphous;  sp.  gr.  3.8).  Upon  these  suppositions  the  alterations 
stand — 

For  zoisite — 

(1)  Ca.,(AK^H)Al2(SiOt)3+2CO.1+3H.iO=2CaCOs+Al(OH),)+H4Al2Si2Oi,+SiO2+k 
and  for  epidote — 

(2)  Ca6(A10H)sAl4Fe2Si9036+6C02+8JH20= 

6CaCOs+3Al(OH)s+2H4Al2Si2O9+Fe2O3.lJH2O+5SiO2+k. 

The  increase  in  volume  of  all  the  compounds  formed  as  compared  with 
the  zoisite  is  66.22  per  cent,  and  as  compared  with  the  epidote  is  69.08  per 
cent. 

Of  course  there  are  many  other  ways  in  which  the  equations  could  be 
written.  All  of  the  aluminum  might  pass  into  gibbsite  or  diaspore  and 
more  quartz  form.  The  iron  may  pass  into  hematite  in  whole  or  in  part, 
etc.  While  all  this  is  true,  it  is  believed  that  the  above  equations  represent 
correctly  the  fundamental  fact  that  by  hydration  and  carbonatiou  zoisite 
and  epidote  in  the  belt  of  weathering  pass  into  simpler  compounds. 

Similar  reactions  could  be  written  for  the  alterations  of  piedmontite 
and  allanite,  but  considering  the  comparative  rarity  of  these  compounds 
this  will  not  be  done. 

AXINITE. 

Axinite: 

HCasAl2BSi4Oili.     (In  some  casea  part  of  the  Ca  is  replaced  by  Fe  and  Mn. ) 

Triclinic. 

Sp.  gr.  3.271-3.294. 

occurrence. — Axiiiite  occurs  as  a  secondary  constituent  in  basic  eruptive 
rocks,  subh  as  the  diabases  and  gabbros.  It  is  found  to  some  extent  in  the 
schists  and  gneisses,  and  particularly  in  those  bearing  abundant  pyroxene 
and  amphibole.  It  also  occurs  in  altered  sedimentary  rocks  as  a  product 


324  A  TREATISE  ON  METAMORPHISM. 

formed  in  connection  with  the  contact  action  of  such  rocks  as  granites, 
granulites,  diabases,  and  gabbros.  In  such  positions  the  formation  of 
axinite  is  usually  regarded  as  assisted  by  fumarole  action. 

Alterations.  —  Apparently  the  alterations  which  axinite  undergoes  in 
rocks  have  not  been  worked  out,  as  they  are  not  recorded  in  the  standard 
text-books. 

PREHNITE. 

Prehnite: 


Orthorhombic. 
Sp.  gr.  2.8-2.95. 

occurrence.  —  Prehnite  is  almost  identical  in  its  occurrence  with  the 
zeolites  (see  pp.  331-333).  It  is  therefore  especially  prevalent  in  the  basic 
and  intermediate  rocks,  such  as  anorthosite,  basalt,  diabase,  gabbro, 
andesite,  diorite,  and  syenite;  also  it  occurs  to  some  extent  in  granites  and 
gneisses,  where  it  may  be  associated  with  epidote.  In  the  igneous  rocks  it 
is  especially  prevalent  in  the  volcanics,  since  these  are  usually  more 
porous.  Like  the  zeolites,  it  is  a  very  frequent  occupant  of  amygdaloidal 
cavities,  and  also  of  cracks  and  crevices  in  the  rocks.  As  already  inti- 
mated, the  most  constant  associates  of  prehnite  are  the  zeolites.  Prehnite 
occurs  to  some  extent  in  the  schists  and  gneisses,  including  those  derived 
from  igneous  rocks,  such  as  the  amphibolites,  and  from  aqueous  rocks, 
such  as  the  marbles.  In  some  cases  it  is  found  in  cavities  in  sedimentary 
rocks  which  have  been  metamorphosed  by  granitic  or  granulitic  intrusions. 
As  a  secondary  mineral  prehnite  is  often  derived  from  aualcite,  laumontite, 
mesolite,  iiatrolite,  the  plagioclases,  and  scolecite.  Fused  prehnite  yields 
wollastonite  and  ankerite. 

Alterations.  —  The  only  alteration  which  I  have  been  able  to  note  in  refer- 
ence to  prehnite  is  to  chlorite  (monoclinic;  sp.  gr.  2.71-2.725).  This 
change  requires  the  substitution  of  magnesium  for  calcium.  Supposing 
the  chlorite  were  amesite  (hexagonal  plates;  sp.  gr.  2.71),  the  change  might 
be  expressed: 

H.,Ca,Al2Si,O12+2MgCO8+H2O=H4MgaAl,SiO9+2SiO2+2CaCOs+k. 

Ignoring  the  carbonates,  the  increase  of  volume  of  the  chlorite  and  quartz 
(rhombohedral;  sp.  gr.  2.6535)  as  compared  with  the  prehnite  is  3.27  per 
cent.  The  change  is  one  of  hydration  and  desilication,  and  would  be 
expected  to  take  place  in  the  zone  of  katamorphism. 


ROCK-MAKING  MINERALS.  325 

HUMITE    GROUP. 
CHONDRODITK,  HUMITE,  AJiD  CLIXOH  U MITE. 

The  humite  group  includes: 

Chondrodite: 

[Mg(F.OH)]2MgsSiA. 

Monoclinic. 

Sp.  gr.  3.1-3.2. 

Humite: 

[Mg(F.OH)]2Mg5SiA3. 

Orthorhombie. 

Sp.  gr.  3.1-3.2. 

Clinohumite: 

[Mg(F.OH)]2Mg7SiA,. 

Monoclinic. 

Sp.  gr.  3.1-3.2. 

In  all  the  above  the  hydroxide  (OH)  replaces  a  part  of  the  fluorine. 

occurrence. — The  humites  occur  in  masses  of  raagnesian  limestones  and 
rocks  bearing  carbonates  ejected  by  volcanoes.  Chondrodite  has  a  some- 
what widespread  occurrence  in  the  marbles  of  eastern  United  States.  In 
such  cases  it  is  sometimes,  at  least,  a  contact  mineral.  Frequently  it  is 
accompanied  by  spinel. 

Alterations. — The  mcst  frequent  alterations  of  the  humites  are  to  serpen- 
tine (monoclinic;  sp.  gr.  2.50-2.65)  and  brucite  (rhombohedral ;  sp.  gr. 
2.38-2.4).  In  the  equations  for  the  alterations  the  hydroxide  will  be  ignored. 
The  reactions  may  be  written  as  follows: 

(1)  (MgF),MgsSiA+3H,0=H,Mg,SiA+Mg(OH).,+MgF,+k. 

(2)  2[(MgF)2Mg5Si,012]  +9H,0=3H4Mg,SiA+3Mg(OH)-,+2MgF,+k. 

(3)  (MgF),Mg7SiA.+6H,0=2H4Mg!)SiA+2Mg(OH  )2+MgF2+k. 

The  increase  in  volume  of  the  serpentine  and  brucite  as  compared  with 
chondrodite,  from  which  it  is  derived,  is  30.15  per  cent;  as  compared  with 
humite,  35.53  per  cent;  as  compared  with  clinohumite,  38.39  per  cent. 

The  alterations  of  humite  to  serpentine  and  brucite  involve  hydration, 
expansion  of  volume,  and  liberation  of  heat.  They  are  therefore  typical 
reactions  of  the  zone  of  katamorphism. 


326  A  TREATISE  ON  METAMOKPHISM. 


TOURMALINE. 


Tourmaline  is  a  complicated  aluminum  silicate,  which  may  be  of  any 
one  of  four  different  types  or  intermolecular  growths  of  these  types. 
According  to  Clarke,  the  formulae  for  these  types  are —  a 

Tourmaline: 

NaHRjAlgBjSi/).,!  (R  in  some  cases  being  lithium  and  hydrogen). 

NaHsMg2Al,B3Si6O3i  (Mg  frequently  being  replaced  by  Fe). 

NaH.M&Al.R.Si.O,,. 

NaH5Mg4Al5B,Si6Osl. 

Rhombohedral. 

Sp.  gr.  2.98-3.20. 

occurrence. — Tourmaline  rather  frequently  occurs  in  the  marbles  and  in 
the  calcareous  schists.  It  also  has  a  rather  widespread  occurrence,  although 
generally  not  as  an  abundant  mineral,  in  granites,  gneisses,  schists,  and 
granulite.  In.  these  rocks  it  frequently  occurs  in  such  relations  to  dikes  of 
igneous  rocks,  especially  of  pegmatites,  as  to  suggest  that  its  development 
is  promoted  by  contact  action.  Because  of  the  boron,  tourmaline  has 
generally  been  regarded  as  evidence  of  fumarole  action.  Certain  it  is  that 
boron  is  not  usually  a  constituent  of  the  ordinary  sediments,  and  to 
account  for  this  element,  especially  where  the  tourmaline  is  abundant,  as  it 
occasionally  is  in  the  schists,  would  seem  to  require  its  introduction  from 
an  outside  source,  either  by  gaseous  or  by  aqueous  solutions. 

Alterations. — Mineral  specimens  of  tourmaline  are  recorded  as  altering 
into  mica,  chlorite  (monoclinic;  sp.  gr.  2.71-2.725),  and  steatite  (massive; 
sp.  gr.  2.794).  However,  in  rocks  tourmaline  is  one  of  the  more  permanent 
minerals,  and  the  chemical  additions  and  subtractions  which  occur  in  the 
alterations  are  so  little  known,  and  the  exact  nature  of  the  tourmaline  from 
which  individual  minerals  are  derived  is  so  uncertain,  that  it  is  not  thought 
advisable  to  attempt  to  write  all  the  reactions  representing  these  changes. 
If  one  assumes  a  definite  tourmaline  and  a  definite  mica  as  being  produced 
from  it,  it  is  easy  to  write  a  reaction.  For  instance,  supposing  that  normal 
biotite  (monoclinic;  sp.  gr.  2.90)  is  derived  from  a  tourmaline  of  the 
composition  of  the  last  of  the  four  formulae  given,  that  the  additional 
alkalies  are  added  in  the  forms  of  carbonates,  that  the  free  boric  acid 

a  Clarke,  F.  W.,  The  constitution  of  the  silicates:  Bull.  U.  8.  Geol.  Survey  No.  125,  1895, 
pp.  56-57.     An  alternative  form  has  been  proposed  by  Penfield. 


'  OCCURRENCE  AND  ALTERATIONS  OF  STAUROLITE.     327 

passes  into  borax,  and  that  the  excess  of  alumina  separates  as  gibbsite 
(monoclinic;  sp.  gr.  2.35),  the  reaction  is — 

4NaH5Mg1Al5B8SiAi+4K.CO,-fNa,CO,= 

8HKMg2Al.,Si3O1.1+3Na.iBjO7+4Al(OH)s+5CO.,+k. 

The  decrease  in  volume  of  the  biotite  as  compared  with  the  tourmaline  is 
6.75  per  cent;  but  if  the  gibbsite  be  included  the  increase  of  volume 
is  3.96  per  cent. 

STAUEOLITE. 

Staurolite: 

HFeAl5Si.2Ols 
Orthorhombic. 
Sp.  gr.  3.65-3.77. 

occurrence. — Staurolite  is  similar  in  its  occurrence  to  garnet,  but  apparently 
requires  more  intense  metamorphic  action  for  it  to  begin  to  form.  Its  most 
widespread  occurrence  is  in  the  schists  and  gneisses  of  sedimentary  origin. 
It  also  develops  in  profoundly  metamorphosed  rocks  of  eruptive  origin,  but 
it  is  not  known  as  an  original  constituent  in  any  eruptive  rock.  Like 
garnet,  it  may  be  abundantly  developed  in  the  zone  of  anamorphism  in 
rocks  which  are  cut  by  intrusives.  The  conditions  favorable  to  its  formation 
are  therefore  similar  to  those  which  produce  garnet  (see  pp.  300-302)  and 
such  minerals  as  tourmaline,  andalusite,  sillimanite,  and  cyanite,  with 
which  it  is  associated.  It  is  evidently  a  mineral  which  derives  its  materials 
from  various  other  minerals,  the  elements  being  recombined  into  the  more 
compact  form  of  Staurolite  under  deep-seated  conditions. 

Alterations. — The  only  alterations  recorded  for  staurolite  are  to  talc 
(orthorhombic  or  monoclinic;  sp.  gr.  2.75),  to  chlorite  (monoclinic;  sp.  gr. 
2.71-2.725),  and  to  muscovite  (damourite)  (monoclinic;  sp.  gr.  2.76—3.0). 
The  first  two  minerals  are  essentially  magnesian  ones,  although  if  the 
chlorite  be  aphrosiderite  (massive;  sp.  gr.  2.90)  a  considerable  amount  of 
iron  may  be  present.  It  is  therefore  clear  that  in  the  change  to  talc  and 
chlorite  magnesium  must  be  derived  from  some  other  compounds.  As  the 
alterations  are  those  which  occur  in  the  zone  of  katamorphism,  it  may  be 
supposed  that  the  magnesium  is  in  the  form  of  carbonate,  since  magnesium 
carbonate  is  an  almost  universal  constituent  of  ground  waters  in  the  upper 
physical-chemical  zone.  The  alteration  to  muscovite  requires  an  entire  loss 
of  the  iron  and  the  addition  of  potassium.  It  is  therefore  clear  that  some 


328  A  TREATISE  ON  METAMORPHISM. 

potassium  mineral  must  also  be  concerned  in  this  alteration.     Staurolite 
rocks  usually  contain  orthoclase,  or  at  least  some  potash  feldspar.     It  may 
be  supposed  that  these  potash  feldspars  break  up  into  kaolin  at  the  same 
time,  thus  furnishing  the  potassium  necessary  for  the  change. 
The  change  of  staurolite  to  talc  may  be  written  as  follows: 

(1)  2HFeAlsSiAs+3MgCO,+15H2O+O=H2MgsSi4O12+FeA+10Al(OH)3+3COJ+k. 

The  decrease  of  the  volume  of  the  talc  as  compared  with  the  staurolite  is 
44.02  per  cent;  but  if  the  gibbsite  (moiioclinic;  sp.  gr.  2.35)  be  included 
the  increase  in  volume  of  the  two  is  90.96  per  cent. 

In  the  change  to  chlorite  the  most  aluminous  one  is  chosen,  amesite 
(hexagonal  plates;  sp.  gr.  2.71),  since  staurolite  is  so  heavily  aluminous 
Moreover,  it  is  supposed  that  the  iron  in  the  chlorite  is  to  the  magnesium  as 
1:3.     On  these  suppositions  the  reaction  may  be  written: 

(2)  2HreAl5Si2O,3+lOH2O+6MgCO,=2H8Mg3FeAl4Si2O18+2Al(OH)3+6COs+k. 

The  increase  of  volume  of  the  chlorite  and  gibbsite  as  compared  with  the 
staurolite  is  103.58  per  cent 

The  change  of  staurolite  to  muscovite  may  be  written: 

(3)  3HFeAl5Si2Ou+K2COs+14H2O+O=2H2KAlsSi8O12+FesO1+9Al(OH),>+CO2+k. 

The  decrease  in  volume  of  the  muscovite  as  compared  with  the  staurolite 
is  24.90  per  cent;  but  if  the  magnetite  (isometric;  sp.  gr.  5.174)  and  gibbsite 
be  included  the  increase  in  volume  would  be  68.08  per  cent,  and  if  hema- 
tite (rhombohedral;  sp  gr.  5.225)  or  limoiiite  (amorphous;  sp.  gr.  3.8) 
form,  instead  of  inagnetite,  the  increase  would  be  still  greater. 

In  the  above  equations  it  is  entirely  possible  that  the  magnesium  may 
be  added  in  some  other  form  than  that  given,  and  the  resultant  compounds 
be  different.  The  same  thing  may  be  said  of  the  potassium.  It  is  uncer- 
tain what  becomes  of  the  excess  of  aluminum.  In  the  equations  the 
aluminum  is  regarded  as  passing  into  the  gibbsite.  However,  the  presence 
of  abundant  gibbsite  is  not  recorded  among  the  alterations  of  staurolite, 
although  frequently  corundum  (rhombohedral;  sp.  gr.  4.025)  occurs  in 
connection  with  it;  but  it  is  by  no  means  certain  that  this  corundum  is  one 
of  the  results  of  the  alteration  of  the  staurolite;  indeed,  it  is  more  probable 
that  the  corundum  formed  from  gibbsite  at  the  same  time  the  staurolite 
developed. 


ZEOLITE  GROUP.  329 

In  short,  the  above  reactions  are  probably  as  unsatisfactory  as  any 
that  have  been  written,  because  the  text-books  do  not  record  what  minerals 
accompany  the  talc,  chlorite,  and  muscovite  as  a  result  of  the  transforma- 
tion of  the  staurolite.  It  is  certain  that  in  each  case  some  other  minerals 
must  be  produced. 

ZEOLITE    GROUP. 

The  zeolites  are  a  great  group  of  hydrous  silicates  about  which  there 
seems  to  be  no  consensus  of  opinion  as  to  the  species  in  the  group,  as  to 
the  composition  of  the  species,  or  as  to  their  classification.  Since  Groth 
and  Clarke  are  among  the  latest  authors  to  discuss  this  group,  their 
formulae  are  used,  Groth's  being  placed  first  and  Clarke's  second  when  he 
differs  from  Groth.  Groth's  formulae  are  put  into  an  empirical  form,  and 
the  subordinate  constituents  which  may  replace  the  chief  bases  are  omitted. 
The  differences  between  the  formula?  given  and  Dana's  also  are  pointed 
out.  The  important  rock-making  zeolites  are  as  follows,  ranged  from  basic 
to  acid: 

THOMSOMTK,  HYDKOSEPHELITE,  1VATBOLITE,  MESOLITE,  SCOLECITE,  ANALCITE,  APOPHYLLITE,  EPISTILBITE, 
HEULAXDITE,  ST1LBITE,  PHILLIPS1TE,  HARMOTOME,  GISMOXDITE,  CHABAZITE,  GMELIMTE,  AND 
LACMOXTITE. 

Thomsonite: 

(CaNa-j)  Al2Si2O8-2iH2°     (Dana  agrees  with  Groth.) 


Orthorhombic. 
Sp.  gr.  2.3-2.4. 

Hydronephelite: 

HNa2Al8Si8O12.3H2O 
HNa2Al3SisOI2.3H,O 
Hexagonal. 
Sp.  gr.  2.263. 

Natrolite: 

Na.,Al2Si3O,0.2H2O     (  Dana  agrees  with  Groth.  ) 

Orthorhombic. 

Sp.  gr.  2.20-2.25. 

Mesolite: 

H2Na2CaAl4Si6O21.4H.!O.     (Dana  makes  all  water  that  of  hydration.  ) 
HgNajCaAl^SijO^.  H  2O. 
Monoclinic  and  triclinic. 
Sp.  gr.  2.29. 


330  A  TREATISE  ON  METAMORPHISM. 

Scoleciie: 

H2CaAl2Si,On.2H2O.     (Dana  agrees  with  Groth. ) 

Monoclinic. 

Sp.  gr.  2.16-2.4. 

Analcite: 

NajAljSiA^HjO.     (Dana  agrees  with  Groth.) 

Isometric. 

Sp.  gr.  2.22-2.29. 

Apophyttite: 

H,KCa4Si8O2«-4iH;1O.     (Dana  agrees  with  Groth. ) 


Tetragonal. 
Sp.  gr.  2.3-2.4. 

Epistilbite: 

inOjB.VHjO.     (Dana  varies  from  both,  but  is  nearer  Groth.     His  formula  is 


Monoclinic. 
Sp.  gr.  2.25. 

Heulandite: 

SHjO.     (Dana  agrees  with  Groth.) 


Monoclinic. 

Sp.  gr.  2.18-2.22. 

StUbtte  (desmine): 

Ca4Al8Si16O48.18H2O.     (Dana's  formula  is  nearly  the  same  as  Clarke's: 

H.tNajCaJAljSieO^HjO.  ) 
Ca,Al,Si,8Ot8.18H.iO. 
Monoclinic. 
Sp.  gr.  2.094-2.205. 

Phillipgite: 

(K2Na2Ca)sAl6Si10032.12H2O.     (Dana's  formula  is  nearly  like  Clarke's: 
(K2Ca)Al.,Si4Ola.4JH2O.) 


Monoclinic. 
Sp.  gr.  2.2. 

Harmotome: 

Ba3Al,Sii0OS2.12H2O.     (Dana  varies  from  each.     His  formula  is  (K2Ba)Al.,Si5OM.5H,O.) 

BasAl6SiuO40.14H2O. 

Monoclinic. 

Sp.  gr.  2.44-2.50. 


OCCURRENCE  OF  ZEOLITES.  331 

Gismondite: 

Ca3Al6Si,O24.12H2O.     (Dana's  formula  is  one  of  the  molecules  given.) 

Ca3Al6Si4O24.12H2O. 

Monoclinic. 

Sp.  gr.  2.265. 

Chabuzite: 

Ca3Al6Si1003;,.16H2O.     (Dana  agrees  with  Clarke  as  to  the  amount  of  Si.     Hia  formula 

is  (  CaXa2)  Al2Si4O12.6H20.  ) 
Ca3Al6Si,.!OS(i.18H.,O. 
Rhombohedral. 
Sp.  gr.  2.08-2.16. 

Gmelinite: 

iujOaj.lBHjO.     (Dana  varies  from  each.    Hia  formula  ia  (Na2Ca)Al2Si4O12.6H2O.  ) 


Rhombohedral. 
Sp.  gr.  2.04-2.17. 

Laumontite: 

'H4CaAljSi4O,4.2H,O.      (Dana  agrees  with  Groth.) 
Ca,Al.Si120»i.12H20. 
Monoclinif. 
Sp.  gr.  2.25-2.3(>. 

occurrence.  —  The  zeolites  are  not  known  as  original  pyrogenic  constitu- 
ents of  igneous  rocks.  As  secondary  minerals  they  are  most  abundantly 
found  in  basic  lavas  and  allied  rocks,  including  both  glassy  and  crystallized 
kinds.  The  zeolites  occur  in  these  rocks,  both  in  the  ordinary  feldspathic 
varieties,  such  as  basalt  and  trachyte,  and  in  those  containing  leucite, 
nephelite,  sodalite,  etc.,  and  especially  in  those  which  are  somewhat  vesicu- 
lar. The  zeolites  also  develop  abundantly  in  the  deep-seated  equivalents 
of  the  basalts  in  such  rocks  as  the  diabases,  gabbros,  etc.  The  zeolites  are 
less  prevalent,  although  far  from  rare,  in  the  diorite  and  syenite  families, 
including  both  the  ordinary  syenites  and  nepheline-syenites.  Finally,  the 
zeolites  are  far  from  uncommon  in  the  more  acid  rocks,  such  as  those  of 
the  granite  family.  In  short,  the  zeolites  may  occur  in  almost  any  variety 
of  the  igneous  rocks,  but,  as  already  said,  are  most  prevalent  in  the  basic 
group.  In  the  sedimentary  rocks  the  zeolites  may  be  allogenic  constituents 
separate  from  other  minerals,  or  an  alteration  product  of  partly  decomposed 
minerals.  Also,  after  the  deposition  of  material  in  the  fragmental  rocks 
the  zeolites  may  develop  as  alteration  products.  Hence  the  zeolites  are 
constituents  of  the  altered  sedimentary  rocks,  of  the  semi  metamorphosed 


332  A  TREATISE  ON  METAMORPHISM. 

sedimentary  rocks,  and  of  the  schists  and  gneisses  of  sedimentary  origin, 
which,  after  becoming  schists  and  gneisses,  have  been  subjected  to  agencies 
of  alteration  in  the  zone  of  katamorphism. 

The  zeolites  develop  from  many  minerals,  but  especially  from  the 
plagioclase  feldspars  and  from  the  leucites,  sodalites,  nephelites,  etc.  From 
the  plagioclases  many  of  the  zeolites  are  produced.  The  following  may  be 
regarded  as  derived  from  anorthite :  Thomsonite,  gismondite,  laumontite, 
phillipsite,  heulaiidite,  epistilbite,  stilbite,  chabazite,  and  scolecite.  The 
following  may  be  regarded  as  derived  from  albite:  Analcite  and  natrolite. 
Mesolite  may  be  regarded  as  derived  from  albite  and  anorthite  together. 
Since  the  intermediate  plagioclases  contain  both  the  anorthite  and  the  albite 
molecules,  all  of  the  above  minerals  may  be  derived  from  oligoclase, 
andesiiie,  labradorite,  and  bytownite,  as  may  also  mesolite.  So  far  as 
recorded  the  derivations  of  the  zeolite  minerals  from  the  nephelites,  leu- 
cites,  and  sodalites  are  as  follows:  Thomsonite  from  nephelite  and  sodalite; 
hydronephelite  from  nephelite  and  sodalite;  natrolite  from  iiephelite, 
sodalite,  haiiynite,  and  noselite;  analcite  from  leucite,  nephelite,  and  soda- 
lite;  stilbite  from  haiiynite  and  noselite;  chabazite  from  haiiynite  and 
noselite.  The  zeolites  are  also  derived  from  other  minerals  as  follows: 
Analcite  from  laumontite,  natrolite  from  apatite  and  chabazite,  etc. 

It  is  hardly  worth  while  to  consider  the  occurrence  of  each  of  the 
zeolites.  It  may  be  said,  however,  that  the  calcium-bearing  zeolites 
are  most  apt  to  form  in  the  calcareous  rocks,  and  the  soda  zeolites  in 
the  rocks  rich  in  soda.  Thus  stilbite,  scolecite,  and  similar  minerals  are 
likely  to  form  in  the  calcareous  rocks  and  limestones,  while  hydronephelite, 
natrolite,  and  analcite,  and  similar  minerals,  are  especially  likely  to  form 
from  the  rocks  containing  soda  feldspars  and  nephelites,  leucites,  and 
sodalites.  The  sodium-calcium  zeolites,  such  as  thomsonite,  mesolite,  and 
phillipsite,  may  occur  in  the  calcareous  rocks,  such  as  the  limestones,  in 
the  igneous  soda  rocks,  such  as  the  nephelite  rocks,  and  in  the  basalts  and 
similar  rocks. 

In  the  rocks  in  which  they  occur  the  zeolites  may  be  found  (1)  within 
the  mass  of  the  rock  as  alteration  products  of  the  minerals;  (2)  in  amyg- 
dules,  filling  the  vacuoles  of  the  igneous  rocks;  and  (3)  in  other  openings 
of  all  kinds,  such  as  fractures,  the  pores  of  sediments,  etc. 


ALTERATIONS  OF  ZEOLITES.  333 

The  development  of  the  zeolites  in  nearly  all  cases  requires  hydration 
and  expansion  of  volume,  as  shown  under  the  discussions  of  the  particular 
minerals  from  which  they  form.  Their  formation,  therefore,  tends  to  fill 
up  the  crevices  and  cracks  in  rocks,  even  if  no  material  be  furnished  from 
an  extraneous  source.  It  may  be  that  the  zeolitization  combined  with  other 
alterations  furnishes  sufficient  material  to  entirely  fill  the  vacuoles  of  many 
arnygdaloids  without  material  being  furnished  from  an  extraneous  source. 
(See  pp.  631-634.)  However,  it  is  doubtless  the  case  that  much  of  the  mate- 
rial of  the  zeolites  which  is  deposited  in  the  belt  of  cementation  is  derived 
by  solution  from  the  belt  of  weathering.  As  shown  under  the  individual 
minerals  from  which  the  zeolites  develop,  the  conditions  for  the  formation 
of  these  minerals  are  those  of  the  zone  of  katamorphism,  both  in  the  belt 
of  weathering  and  in  that  of  cementation. 

In  the  belt  of  cementation,  in  which  hydration  is  perhaps  the  most 
characteristic  reaction  and  alterations  can  take  place  with  expansion  of 
volume,  the  zeolites  form  on  a  great  scale.  Conforming  with  these  state- 
ments are  the  observations  made  by  Daubrde"  that  zeolites  can  be  formed 
experimentally  in  the  presence  of  abundant  water  at  temperatures  of  about 
50°  C.  Pointing  in  the  same  direction  is  the  fact  stated  by  Renard6  that 
phillipsite  has  extensively  formed  at  the  bottom  of  the  sea  at  temperatures 
not  far  from  0°  C. 

While  the  zeolites  develop  chiefly  in  the  belt  of  cementation,  it  is 
certain  that  in  very  humid  regions  they  form  in  the  belt  of  weathering. 
But  it  is  also  certain  that  to  a  great  extent  the  zeolites  are  also  destroyed  in 
the  belt  of  weathering.  This  is  especially  the  case  in  hot  arid  regions. 

Alterations. — The  most  comprehensive  statement  as  to  the  alterations  of 
the  zeolites  is  that  given  by  Clarke/  His  statements  are  as  follows:  (1) 
Natrolite  alters  into  prehnite  (orthorhombic;  sp.  gr.  2.875);  (2)  mesolite 
alters  into  prehnite;  (3)  scolecite  alters  into  prehnite;  (4)  analcite  alters 
into  albite  (triclinic;  sp.  gr.  2.635),  and  (5)  orthoclase  (monoclinic;  sp.  gr. 
2.57)  and  prehnite;  (6)  apophyllite  alters  into  pectolite  (monoclinic; 
sp.  gr,  2.73);  (7)  heulandite  alters  into  albite,  and  (8)  into  orthoclase; 

«Daubr£e,  A.,  fitudes  synthotiquea  de  geologic  expe>imentale,  Paris,  1879,  pt.  1,  pp.  199,  205-207. 
*  Murray,  John,  and  Renard,   A.  F.,  Report  of  the  scientific  results  of  the  voyage  of  H.  M.  S. 
Challenger,  1873-1876;  Deep-sea  deposits,  London,  1891,  pp.  400-411. 

c  Clarke,  F.  W.,  The  constitution  of  the  silicates:  Bull.  U.  S.  Geol.  Survey  No.  125,  1895,  pp.  32--15. 


334  A  TREATISE  ON  METAMORPH1SM. 

(9)  stilbite  alters  into  albite,  and  (10)  into  orthoclase;  (11)  ehabazite 
alters  into  natrolite  (hexagonal-rhombohedral  (Hintze),  orthorhombic 
(Dana);  sp.  gr.  2.225);  (12)  laumontite  alters  into  albite,  (13)  into  ortho- 
clase and  prehuite,  and  (14)  into  analcite  (isometric;  sp.  gr.  2.255). 

These  alterations  may  be  expressed  by  the  following  equations,  the 
numbers  of  the  equations  corresponding  to  the  numbers  of  the  alterations. 
In  writing  these  equations,  whether  Groth's  or  Clarke's  formula  is  used 
depends  on  which  is  more  nearly  analogous  to  the  formula  of  the  mineral 
produced. 

(1)  Na2Al2Si,010.2H20+2CaC03=H.,Ca2AL1Sis012+Na.1C03+C02+H20-k. 

(2)  H2Na2CaAl4Si602i.4H2O+3CaCOs=2H2Ca,Al2Si3O12+Na2C08+2CO2+3H2O-k. 

(3)  H2CaAl2SisO1I.2H2O-rCaCOs=H2Ca2Al.!Si3O12+2H20+CO2-k. 

(4)  Na2Al2Si4012.2H2O+2SiO2=2NaAlSisO8+2H2O-k. 

(5)  3(Na2Al2Si4012.2H2O)+4CaCO3+K2C08= 

2KAlSi3O8+2H2Ca2Al2Si3O12+3Na2CO3+2CO2+4H2O-k. 

(6)  H,4Ca4Si6O23+Na2C03=2HNaCa,Si3O9+6H2O+CO2--k. 

(7)  H4CaAl2Si6O18.3H2O+Na2C08=2NaAlSi3O8  +  CaCO3^5H2O-k. 

(8)  H4CaAl2Si6O,8.3H2O+K2CO3=2KAlSi308-rCaCO3+5H2O-k. 

(9)  Ca3Al6Si18O48.18H2O+3Na2CO3=6NaAlSisO8+3CaCO3+18H.iO— k. 

(10)  CasAl6Si18O48.18H2O+3K2CO3=6KAlSisO8+3CaCO3+18H.!O-k. 

(11)  Gas  Al.SiuO,,.  18H2O+ 2  Al  (OH )  8+4Na.,COs = 

4H4Na2Al2Si8O12r3CaCO8+COj+13H2O-k. 

(12)  Ca3Al6Si12O36.12H2O+2Na2CO5+CO2=4NaAlSi8O8+Al2O3+3CaCO9+12H2O-k. 

(13)  Ca3Al6Si,2OS6.12H2O-rCaOO3+K2CO3= 

2KA]Si3O8+2H2Ca2Al28i3O12+2CO2+10H2O-k. 

(14)  Ca3Al6Si,2OS6.12H2O+3Na2C03=3(NaiAl2Si4012.2H2O)+3CaCO3i-6H2O-k. 

The  decreases  in  volumes  are  as  follows:  Prehnite  as  compared  with 
the  natrolite,  equation  (1),  16.12  per  cent;  prehnite  as  compared  with  meso- 
lite,  equation  (2),  15.05  per  cent;  prehnite  as  compared  with  scolecite, 
equation  (3),  16.66  per  cent;  albite  as  compared  with  the  analeite  and 
quartz,  equation  (4),  17.25  per  cent;  orthoclase  and  prehnite  as  compared 
with  analcite,  equation  (5),  14.09  per  cent;  pectolite  as  compared  with 
apophyllite,  equation  (6),  19.48  per  cent;  albite  as  compared  with  heulan- 
dite,  equation  (7),  25.03  per  cent;  orthoclase  as  compare.d  with  heulandite, 
equation  (8),  18.44  per  cent;  albite  as  compared  witli  stilbite,  equation  (9), 
31.67  per  cent;  orthoclase  as  compared  with  stilbite,  equation  (10),  25.66 
per  cent;  natrolite  as  compared  with  ehabazite,  equation  (11),  4.58  per 


ALTERATIONS  OF  ZEOLITES.  335 

cent;  albite  as  compared  with  laumontite,  equation  (12),  34.92  per  cent; 
orthoclase  and  prelmite  as  compared  with  laumontite,  equation  (13),  17.75 
per  cent;  and  analcite  as  compared  with  laumontite,  equation  (14),  4.30  per 
cent. 

In  calculating  the  volume  relations  the  carbonates,  and  in  equation 
(11)  the  aluminum  hydrate,  are  ignored.  If  these  compounds  were  taken 
into  account  the  decreases  in  volume  would  in  some  cases  be  somewhat 
more,  in  others  somewhat  less.  To  ignore  these  side  compounds  seems  the 
best  course,  since  the  added  and  subtracted  salts  may  be  in  other  forms 
than  those  given. 

For  the  most  part  the  alterations,  so  far  as  the  bases  are  concerned, 
are  remarkably  simple,  involving  only  the  interchange  between  the  alkalies, 
sodium  and  potassium,  or  between  sodium  and  the  alkaline  earth  calcium, 
or  the  addition  of  the  bases  sodium  or  calcium.  They  are  all  reactions  of 
dehydration,  partial  or  complete.  Many  of  them  are  reactions  of  decar- 
bonation  and  one,  equation  (4),  is  a  reaction  of  silication.  Presumably 
they  all  take  place  with  the  absorption  of  heat.  While  nothing  can  be 
ascertained  as  to  the  actual  conditions  under  which  the  changes  take  place, 
one  would  expect  them  to  occur  in  the  zone  of  anamorphism,  for  a  number 
of  the  reactions  reverse  those  of  the  zone  of  katamorphism.  In  the  latter 
zone  the  alteration  of  the  feldspars  into  the  zeolites  is  well  known.  The 
reverse  changes,  those  of  analcite,  heulandite,  stilbite,  and  laumontite  into 
albite  and  orthoclase,  for  which  equations  are  written,  would  be  hardly 
likely  to  take  place  in  the  same  zone.  At  any  rate  the  alterations  of  the 
feldspars  into  the  zeolites  and  the  zeolites  into  the  feldspars  present  a  very 
interesting  case  of  reversible  reactions  discussed  subsequently.  (See 
pp.  366-369.) 

It  is  further  certain  that  the  zeolites  as  extensively  formed  in  the  belt 
of  cementation,  in  the  belt  of  weathering  break  up,  by  carbonation  and 
hydration,  into  the  simpler  compounds,  such  as  the  carbonates  of  the 
alkalies  and  the  alkaline  earths,  diaspore  or  gibbsite,  kaolin,  and  quartz. 
Hypothetical  reactions  could  readily  be  written  for  these  changes  similar 
to  those  worked  out  for  zoisite  and  epidote  (pp.  322-323);  but  since  so 
little  is  known  as  to  the  definite  minerals  which  are  formed  from  each 
zeolite,  this  is  hardly  worth  while  at  the  present  stage  of  knowledge. 


336  A  TREATISE  ON  METAMORPHISM. 

MICA   GROUP. 
MUSCOVITE,   PABAC10MTE,  BIOTITE,  AXD  PHLOdOPITE. 

The  mica  group  includes  the  following  rock- making  species: 

Muscovite: 

(H,K)  AlSiO4.     (Normal  muscovite  KH2Al3SisOu. ) 

Monoclinic. 

Sp.  gr.  2.76-3.0. 

Paragcmite: 

H2NaAl3SisOls. 

Monoclinic. 

Sp.  gr.  2.78-2.90. 

Biotite: 

(H,K)2(MgFe)2Al2Si3O12.     (Dana.)     (Proportion  of  Mg:Fe  varies   widely.     Normal 

biotite:  KHMgiAljSisO^.     (Clarke.)) 
Monoclinic. 
Sp.  gr.  2.7-3.1. 

Phlogopite: 

KH2Mg,AlSisOu- 

Monoclinic. 

Sp.  gr.  2.78-2.86. 

MUSCOVITE. 

Muscovite,  as  already  noted,  is  hydrogen-potassium-aluminum  silicate. 

occurrence. — Muscovite  is  an  abundant  constituent  in  the  plutonic  rocks, 
but  is  rather  rare  as  a  constituent  in  the  volcanic  rocks.  It  is  one  of  the 
most  abundant  constituents  of  the  metamorphosed  rocks,  being  a  chief 
mineral  in  many  metamorphosed  sedimentary  and  many  metamorphosed 
igneous  rocks.  As  a  secondary  constituent,  it  is  derived  from  many  other 
minerals.  The  more  important  of  these  are  feldspar,  including  both  ortho- 
clase  and  plagioclase,  nephelite,  sodalite,  leucite,  the  scapolites,  spodumene, 
topaz,  andalusite,  and  cyanite.  It  is  also  recorded  as  a  pseudomorph  after 
tourmaline,  garnet,  beryl,  and  cordierite.  There  is  little  doubt  also  that 
muscovite  in  the  metamorphosed  rocks  is  largely  formed  from  the  materials 
of  the  zeolites.  Some  of  the  minerals,  such  as  nephelite,  sodalite,  and 
leucite,  from  which  the  muscovite  is  derived,  occur  only  in  the  igneous  rocks. 
Others  of  them,  such  as  the  zeolites,  occur  only  in  rocks  of  altered  or  sec- 
ondary nature.  Others  of  the  minerals  from  which  muscovite  is  derived, 
such  as  topaz,  cyanite,  and  andalusite,  are  chiefly  metamorphic  constituents. 


ALTERATIONS  OF  MUSCOVITE.  ,337 

Still  others  from  which  muscovite  is  derived,  such  as  the  feldspars,  may 
be  original  constituents  of  the  igneous  rocks,  or  they  may  be  original  or 
secondary  constituents  of  the  sedimentary  rocks.  It  is  therefore  clear  that 
muscovite  has  an  unusual  variety  of  sources,  and  consequently  it  may  be 
expected  in  almost  any  variety  of  rock  except  the  volcanics.  It  is,  how- 
ever, a  more  characteristic  constituent  of  the  acidic  and  intermediate  rocks 
than  of  the  basic  rocks. 

In  summary,  muscovite  is  derived  from  anorthoclase,  diaspore,  gibbsite, 
leucite,  microcline,  nephelite,  orthoclase,  plagioclase  and  orthoclase,  scapo- 
lites,  sodalite,  and  spodumene.  The  muscovite  damourite  is  derived  from 
andalusite,  corundum,  cyanite,  sillimanite,  staurolite,  and  topaz. 

Alterations. — The  minerals  to  which  muscovite  alters  are  not  nearly  so 
abundant  as  those  from  which  it  is  derived.  One  of  the  most  frequent 
alterations  is  that  of  hydration,  a  part  of  the  potassium  being  replaced  by 
hydrogen;  or  at  the  same  time  it  may  take  up  other  bases  and  thus  the 
mineral  may  pass  into  vermiculite,  a  somewhat  indefinite  compound  to 
which  no  formula  can  be  assigned.  Muscovite  also  alters  into  serpentine 
(monoclinic;  sp.  gr.  2.50-2.65)  and  into  the  steatitic  form  of  talc  (massive; 
sp.  gr.  2.7-2.8).  Probably  simultaneously  with  the  formation  of  these 
minerals  gibbsite  (monoclinic;  sp.  gr.  2.3-2.4)  or  diaspore  (orthorhombic ; 
sp.  gr.  3.3—3.5)  forms,  although  the  contemporaneous  formation  of  these 
minerals  is  not  mentioned.  Muscovite  also  may  alter  into  the  soda-mica 
paragonite  (monoclinic;  sp.  gr.  2.78—2.90). 

The  reactions  by  which  muscovite  passes  into  serpentine  and  talc  are 
very  uncertain.      If  the  magnesium  were  supposed  to  be  derived  from  a 
carbonate  and  all  of  the  silica  of  the  muscovite  went  into  the  resultant 
compounds,  the  reactions  may  be  written  as  follows: 
For  serpentine: 

(1)  2KH2AlsSi3O12+9MgCO3+13H2O=3H4MgsSi2O9+6Al(OH)s+KaCO8+8CO2+k. 

For  talc : 

(2)  4KH2AlsSi3O12+9MgCO,+17H2O=3H.!Mg3Si4012+12Al(OH)3+2K2CO9+7CO2+k. 

The  increase  in  volume  of  the  serpentine  as  compared  with  the  muscovite  is 
16.56  per  cent,  and  the  decrease  of  the  talc  25.23  per  cent.  But  if  the 
magnesium  carbonate  be  contributed  by  solutions,  and  the  gibbsite  remains 
as  a  solid  with  the  serpentine  and  talc,  the  increase  in  volume  of  the  ser- 
MON  XLVII — 04— — 22 


338  A  TREATISE  ON  METAMORPHISM. 

pentine  and  gibbsite  as  compared  with  the  muscovite  is  88.44  per  cent,  and 
of  the  talc  and  gibbsite  46.69  per  cent. 

The  change  of  muscovite  to  paragonite  merely  requires  the  substitution 
of  sodium  for  potassium,  and  may  be  written  as  follows : 

(3)     2H2KAlsSiAa+NaICOs=2H2NaAl,Si3O12+K2COs+k. 

The  decrease  in  volume  is  2.67  per  cent. 

Muscovite  under  deep-seated  conditions  is  a  mineral  which  is  practi- 
cally permanent.  In  fact,  under  these  conditions,  as  already  indicated,  it  is 
produced  by  the  alteration  of  other  minerals.  The  above  alterations  of 
muscovite,  resulting  in  the  formation  of  vermiculite,  serpentine,  and  talc, 
with  gibbsite  all  occur  in  the  zone  of  katamorphism,  and  especially  in  the 
belt  of  weathering.  Even  under  the  conditions  of  the  surface  belt  the  proc- 
esses of  change  are  exceedingly  slow.  Corresponding  with  this  position, 
the  changes  take  place  with  increase  of  volume  and  liberation  of  heat. 


PARAGONITE. 


Paragonite  is  hydrous  sodium-aluminum  silicate. 

occurrence. — Paragonite  is  not  certainly  known  as  an  original  pyrogenic 
constituent  in  igneous  rocks.  It  is  found  especially  in  the  metamorphosed 
igneous  rocks  and  in  the  semimetamorphosed  and  completely  metamor- 
phosed sedimentary  rocks.  In  many  so-called  sericite  rocks  it  is  probable 
that  a  portion  of  the  micaceous  mineral  is  paragonite  rather  than  muscovite. 
Paragonite  is  especially  likely  to  occur  in  the  metamorphic  rocks,  instead 
of  muscovite,  where  the  original  rocks,  either  igneous  or  sedimentary,  bear 
a  considerable  amount  of  sodium.  Very  frequently  associated  with  para- 
gonite are  the  heavy  metamorphic  minerals,  such  as  cyauite,  staurolite, 
garnet,  tourmaline,  etc.  In  certain  places  muscovite  has  been  noted  as  pass- 
ing to  paragonite,  and  thus  the  potassium  mica  is  a  source  for  the  soda  mica. 

In  summary,  paragonite  as  a  metamorphic  mineral  is  derived  from 
anorthoclase,  muscovite,  and  plagioclases. 

Alterations. — Alterations  of  paragonite  are  not  recorded  in  the  standard 
text-books.  However,  there  can  be  little  doubt  that  this  mineral  undergoes 
a  set  of  alterations  in  the  zone  of  katamorphism,  and  one  would  expect 
that  these  alterations  would  be  analogous  to  those  which  take  place  with 
muscovite. 


MICA  GROUP.  339 


Biotite  is  hydrogen-potassium-magnesium-aluminum  silicate,  a  part  of 
the  magnesium  frequently  being-  replaced  by  iron. 

occurrence. — Biotite  is  an  original  chief  constituent  of  many  of  the  igneous 
rocks,  both  plutonic  and  volcanic,  and  ranging  from  those  which  are  p,cid 
to  those  which  are  basic.  It  is  a  very  abundant  secondary  constituent 
in  the  slates,  schists,  and  gneisses,  developing  on  a  great  scale  in  the 
metamorphosed  rocks,  both  igneous  and  sedimentary.  As  a  secondary 
constituent  it  seems  usually  not  to  be  derived  from  a  single  mineral,  as  is 
frequently  the  case  with  muscovite,  but  is  produced  from  material  furnished 
by  two  or  more  minerals.  For  instance,  it  is  frequently  a  reaction  product 
between  magnetite  and  other  minerals,  the  magnetite  furnishing  the  iron 
for  the  biotite,  the  other  constituents  being  derived  from  such  minerals  as 
the  pyroxenes,  arnphiboles,  and  feldspars.  A  frequent  case  is  the  formation 
of  biotite  from  the  pyroxenes,  feldspars,  and  magnetite.  The  feldspars  and 
feldspathoids  frequently  furnish  the  potassia,  parts  of  the  alumina,  and  silica. 
The  pyroxenes  and  amphiboles  frequently  furnish  a  part  of  the  magnesia, 
alumina,  and  silica.  Dolomite  is  often  a  source  of  the  magnesia.  The 
oxides  and  carbonate  of  iron  are  the  most  frequent  sources  of  this  element. 

In  summary,  as  a  metamorphic  mineral,  biotite  is  derived  from  anortho- 
clase,  augite,  hornblende,  microcline,  orthoclase,  and  the  scapolites. 

Alterations. — Perhaps  the  most  frequent  alterations  of  biotite  are  to 
hydrobiotite  (probably  monoclinic;  sp.  gr.  2.90,  average  of  biotite)  and  to 
chlorite  (monoclinic;  sp.  gr.  2.80).  It  also  alters  into  epidote  (monoclinic; 
sp.  gr.  3.25-3.50);  rarely  it  alters  into  hypersthene  (orthorhombic;  sp.  gr. 
3.40-3.50)  and  sillimanite  (orthorhombic;  sp.  gr.  3.23-3.24);  and  in  some 
cases  it  apparently  alters  into  serpentine  (monoclinic;  sp.  gr.  2.575).  Its 
alteration  into  the  above  minerals  may  be  accompanied  by  the  separation 
of  quartz  (rhombohedral ;  sp.  gr.  2.6535),  and  if  the  biotite  be  ferriferous, 
by  the  formation  of  magnetite  (isometric;  sp.  gr.  5.174),  or  other  iron  oxide. 

The  alteration  of  biotite  into  serpentine  probably  requires  the  simulta- 
neous production  of  kaolin  (monoclinic;  sp.  gr.  2.615)  and  gibbsite  (mono- 
clinic;  sp.  gr.  2.35).  Supposing  that  all  the  magnesium  of  normal  biotite 


340  A  TREATISE  ON  METAMOKPHISM. 

goes  into  the  serpentine,  and  that  all  the  silica  not  required  for  the  pro- 
duction of  this  mineral  passes  into  the  kaolin,  the  reaction  is  as  follows  : 


(1) 

4H4Mg3Si2O9+5H,Al2Si2O9+2Al(OH)8+3K2UO3Tk. 

Supposing  all  the  serpentine,  kaolin,  and  gibbsite  to  remain  as  solids,  and 
the  potassium  carbonate  to  go  into  solution,  the  increase  in  volume  is  14.26 
per  cent. 

The  change  of  biotite  into  hydrobiotite  may  be  written: 

(2)  2HKMg2Al2SisO12+7H2O+CO2=2(H.,Mg2Al2Si8OI2.3H2O)+K.1COs+k. 

The  increase  in  volume  is  3.8  per  cent. 

The  alteration  into  chlorite,  supposing  all  the  alumina  and  silica 
to  remain  in  the  altered  mineral,  and  the  additional  magnesia  to  be  added 
in  the  form  of  a  carbonate,  may  be  written  as  follows  : 

(3)  2KHMg2Al2Si3O12+4MgCOs+5H8O=2[H.!Mg4Al2Si3O1:1--l(OH)]+K2COs+3CO2+k. 

The  increase  in  volume,  supposing  the  magnesium  carbonate  is  added  in 
solution  and  the  potassium  carbonate  goes  into  solution,  is  22.92  per  cent. 
The  reaction  by  which  biotite  passes  into  epidote  is  uncertain.  If  the 
ferrous  iron  of  the  biotite  be  changed  to  sesquioxide  during  the  alteration, 
and  if  the  proportion  of  magnesium  to  iron  be  supposed  to  be  3:1  in  the 
biotite,  and  of  aluminum  to  iron  4:1  in  the  epidote,  the  reaction  may  be 
written  : 

(4)  6H2K8Mg3FeAl4Si6O24+20CaCOs+4CO.1+3O= 

2(H5Ca10Al12Fe8Si15O65)+6SiO2+18MgCO8+6K2CO3+H2O+k. 

The  ratios  assumed  of  the  magnesium  and  iron  for  the  biotite,  and  of 
aluminum  and  iron  for  the  epidote,  are  near  means.  If  it  be  assumed  that 
the  iron  of  the  biotite  is  not  changed  to  sesquioxide,  but  that  the  sesquioxide 
of  iron  for  the  epidote  must  be  derived  from  another  source,  the  reaction 
takes  a  very  different  form.  Under  such  an  assumption  the  epidote  may 
be  produced  from  normal  biotite,  and  the  equation  stand  as  follows: 

(5)  30KHMg2Al2SisO12+6Fe,O,+40CaCOs+35CO,  ->-  H2O= 

4(H5Ca10Al12FesSi16O65)+30SiO:i+12AlO(OH)+60MgCO3+15K2COa+k. 


ALTERATIONS  OF  BIOTITE.  341 

By  adding  thirteen  molecules  of  water  instead  of  one,  twelve  molecules  of 
gibbsite  instead  of  twelve  molecules  of  diaspore  (orthorhombic ;  sp.  gr.  3.40) 
will  be  produced. 

In  equation  (4),  supposing  the  calcium  carbonate  to  be  added  in 
solution  and  the  magnesium  carbonate  and  the  potassiuin  carbonate  to  be 
removed  in  solution  and  the  silica  to  remain  as  a  solid,  the  decrease  in 
volume  is  14.86  per  cent. 

In  equation  (5),  supposing  the  biotite  and  iron  oxide  to  be  solids,  the 
calcium  carbonate  to  be  added  in  solution,  the  magnesium'  and  potassium 
carbonates  to  remain  in  solution,  but  the  epidote,  silica,  and  diaspore  to 
remain  as  solids,  the  decrease  in  volume  is  18.45  per  cent.  If  gibbsite 
instead  of  diaspore  be  produced  the  decrease  in  volume  will  not  be  so  much. 

If  it  be  supposed  that  the  aluminum  passes  into  spinel  (isometric;  sp. 
gr.  3.8)  instead  of  diaspore  or  gibbsite,  and  spinel  is  known  to  form  in  con- 
nection with  biotite,  the  number  of  molecules  of  magnesium  carbonate 
would  be  reduced  by  six  in  equation  (4);  that  is,  to  54.  6MgALO4  would 
replace  the  12A1O(OH).  No  water  would  need  to  be  added,  and  five 
molecules  of  water  would  be  produced.  Finally,  only  twenty-nine  mole- 
cules of  CO2  would  need  to  be  added.  Therefore  the  equation  would  be: 

(6)     30KHMg2Al2Si30I2+6FejOs+40CaCOs+29CO2= 

4H6Ca10Al12FesSi15065+30SiO2+6MgAl2O4+54MgCO3-M5K2CO,^5H,O+k. 

In  this  case  the  volume  of  the  resultant  epidote,  spinel,  and  silica,  would 
be  14.71  per  cent  less  than  that  of  the  biotite  and  18.15  per  cent  less  than 
that  of  the  biotite  and  hematite. 

It  is  a  well-known  fact  that  chlorite  secondary  to  biotite  is  usually 
accompanied  by  epidote  and  quartz.  Comparing  the  equation  (3)  for  the 
formation  of  chlorite  with  equation  (4)  for  epidote,  we  see  why  these  two 
minerals  with  quartz  are  frequently  formed  at  the  same  time.  For  the 
formation  of  chlorite  from  biotite  additional  magnesium  is  needed.  For 
the  formation  of  epidote  additional  calcium  is  necessary  and  magnesium  is 
left  over.  If  instead  of  magnesium  and  calcium  carbonates  being  added, 
as  suggested  in  equations  (3)  and  (4),  only  calcium  carbonate  were  avail- 
able, the  excess  of  magnesium  produced  by  the  passage  into  epidote  may  go 
into  the  chlorite,  and  thus  epidote  and  chlorite  be  simultaneously  produced. 


342  A  TREATISE  ON  METAMORPH1SM. 

Combining  these  equations,  (3)  and  (4),  and  supposing  the  iron  oxide  to  be 
furnished  by  hematite,  the  reaction  may  be  written  as  follows : 

(7)  60KHMg2Al2SisO12+6FeA-40CaOO3-r76H2O  = 

30[H2Mg4Al2Si3O12.4(OH)]+4(H5Ca10Al12Fe3Si15O65)+30SiO.!+12AlO(OH)  + 

30K2OO3+10CO2-k. 

The  increase  in  volume  of  the  epidote,  chlorite,  quartz,  and  diaspore 
together,  as  compared  with  the  biotite  and  hematite,  would  be  1.81  per 
cent. 

If  biotite  alters  to  hypersthene  and  sillimanite  it  may  be  presumed,  in 
order  to  furnish  the  necessary  iron  for  the  hypersthene,  that  the  biotite  is 
an  iron-bearing  one.  If  the  magnesium  be  to  the  iron  as  3:1,  and  the 
hypersthene  be  one  in  which  the  same  ratio  prevails,  the  reaction  may  be 
written  as  follows: 

(8)  H2K2Mg3FeAl4Si6O2rrCO,=Mg3FeSi4O]2+2Al2SiO5+H2O+K2OO3+k. 

The  decrease  in  volume  of  the  hypersthene  and  sillimanite,  as  compared 
with  the  biotite,  is  24.68  per  cent. 

In  the  majority  of  the  above  reactions  a  formula  for  biotite  is  used 
which  contains  no  iron.  The  majority  of  biotites  in  nature  do  contain 
some  iron.  If  this  material  be  present,  simultaneously  with  the  formation 
of  other  minerals  magnetite  (isometric;  sp.  gr.  5.174),  hematite  (rhombohe- 
dral;  sp.  gr.  5.225),  and  the  other  oxides  of  iron  may  be  produced.  The 
abstraction  of  the  iron  oxides  is  accompanied  commonly  by  a  change  in 
color  of  the  altering  biotite  from  brown  to  green.  The  presence  of  these 
compounds,  however,  in  subordinate  quantities  will  not  alter  the  main 
conclusions  as  to  the  volume  relations  above  given. 

The  alterations  of  biotite,  serpentine,  kaolin,  and  gibbsite,  into  hydro- 
biotite  and  chlorite,  equations  (1)  to  (3).  are  all  reactions  which  are 
known  to  occur  in  the  zone  of  katamorphism,  corresponding  with  which 
position  they  are  all  reactions  of  hydration,  and  the  first  two  also  of  car- 
bonation.  Where  chlorite  and  epidote  form  together,  the  reaction  is  that 
of  hydration,  and  doubtless  this  change  also  takes  place  in  the  zone  of 
katamorphism.  The  formation  of  hypersthene  and  sillimanite  from  biotite 
usually  occurs  in  connection  with  contact  reactions  of  igneous  rocks ;  it  is, 
therefore,  a  reaction  requiring  high  temperature.  Also  the  minerals 


OCCURRENCE  AND  ALTERATIONS  OF  PHLOGOPITE.  343 

sillimaiiite  and  hypersthene  do  not  form  at  the  surface,  but  at  depth. 
Corresponding'  with  these  physical-chemical  facts,  the  reaction  is  one  of 
dehydration  and  reduction  of  volume. 


PHLOGOPITE. 


Phlogopite  is  potassiuin-hydrogen-maguesium-aluminum  silicate. 

occurrence. — Phlogopite  has  an  occurrence  which  is  somewhat  different 
from  that  of  biotite.  It  is  especially  characteristic  of  metamorphosed 
impure  carbonates,  such  as  dolomitic  marbles.  In  these  rocks  it  is  often 
associated  with  pyroxene,  amphibole,  etc. 

Alterations. — The  most  frequent  alterations  of  phlogopite  are  to  hydro- 
phlogopite  (monoclinic;  sp.  gr.  2.303,  kerrite)  and  chlorite  (monoclinic; 
variety  of  penninite;  sp.  gr.  2.649).  It  is  also  said  to  alter  to  talc  (orthor- 
hombic  or  monoclinic;  sp.  gr.  2.7-2.8).  In  these  last  two  alterations 
gibbsite  (monoclinic;  sp.  gr.  2.3—2.4)  or  diaspore  (orthorhombic;  sp.  gr.  3.40) 
must  simultaneously  separate. 

The  reactions  for  these  changes  are  as  follows: 

For  hy drophlogopite : 

(l)2H2KMg3AlSi3O12+7H2O+CO,=2(H3Mg3AlSi3O12.3H2O)+K2C03+k. 
For  chlorite: 

(2)2H2KMg3AlSi3O12+6MgCO!>+7H,O=2[H3Mg6AlSi3O12.6(OH)]+K2CO,+5CO2+k. 
For  talc  and  gibbsite: 

(3)4H2KMg3AlSi3012+6H20+-tC02=3H2Mg3SiA2+4Al(OH)3+3MgC03+K:iC03+H20+k. 
(4)2H2KMg3AlSi30I2+C02+4H20=H2Mg3Si40I2+H4Mg3Si209+2Al(OH)3+K.1C03+k. 

In  equations  (3)  and  (4)  if  diaspore  instead  of  gibbsite  were  produced 
less  water  would  be  needed. 

Disregarding  the  carbonates,  the  increase  in  volume  for  hydrophlogo- 
pite,  equation  (1),  is  26.89  per  cent;  for  chlorite,  equation  (2),  is  41.02 
per  cent.  The  decrease  for  talc  and  gibbsite,  equation  (3),  is  7.79  per 
cent,  and  for  talc  and  diaspore  18.27  per  cent.  The  increase  in  volume  of 
the  serpentine,  talc,  and  gibbsite,  equation  (4),  is  5.23  per  cent. 

All  of  these  reactions  are  those  of  hydration  and  solution.  They  are 
characteristic  of  the  zone  of  katamorphism. 


344  A  TREATISE  ON  METAMOKPHISM. 

According  to'  Clarke,  penninite,  one  of  the  chlorites,  is  composed  of 
one  molecule  of  biotite-chlorite  and  one  molecule  of  phlogopite-chlorite; 
and  clinochlore  is  composed  of  two  molecules  of  biotite-chlorite  and  one 
molecule  of  phlogopite-  chlorite.  It  is  therefore  easy  to  combine  the  equa- 
tion given  under  biotite  for  the  production  of  chlorite  with  the  one  under 
phlogopite  producing  chlorite,  and  thus  produce  penninite  (pseudorhombo- 
hedral  and  monoclinic;  sp.  gr.  2.6—2.85)  and  clinochlore  (monoclinic;  sp. 
gr.  2-2.5).  However,  as  the  alterations  for  the  production  of  chlorite  from 
biotite  and  of  chlorite  from  phlogopite,  reactions  of  hydration,  carbona- 
tion,  and  liberation  of  heat  occur  in  the  zone  of  katamorphism,  it  may 
be  said  that  where  penninite  and  clinochlore  are  produced  from  biotite  and 
phlogopite  the  physical-chemical  reactions  are  of  the  same  class  as  those 
which  have  been  given  for  hydrobiotite  and  hydrophlogopite. 

CLINTONITE    GROUP. 
MAROARITK,  i  II  I.OIM1OI  n.  AND  OTTRKUTK. 

The  clintonite  group  includes  the  following  rock-making  minerals: 

Margarite: 

H2CaAl4SisOls. 

Monoclinic. 

Sp.  gr.  2.99-3.08. 

Chlariloid: 

H2(MgFe)Al2Si07. 

Monoclinic  (G)  or  triclinic  (D). 

Sp.  gr.  3.52-3.57. 

Ottrelite: 

H2(FeMn)Al2Si2O9. 
Monoclinic  or  triclinic. 
Sp.  gr.  3.3. 

occurrence.— The  most  common  development  of  margarite  is  in  connection 
with  corundum.  In  a  number  of  cases  it  is  recorded  that  the  alumina  of  the 
margarite  is  directly  furnished  by  the  corundum.  Margarite  is  also  found 
as  a  metamorphic  mineral  in  schists  and  gneisses,  associated  with  the  heavy 
minerals  staurolite,  tourmaline,  etc.  As  a  metamorphic  mineral,  margarite 
is  also  recorded  as  being  derived  from  diaspore  and  gibbsite. 


CHLORITE  GROUP.  345 

Chloritoid  and  ottrelite  both  occur  in  the  slates,  schists,  and  gneisses 
which  are  derived  from  the  argillaceous  sediments  as  a  product  of  or 
connected  with  deep-seated,  and  especially  deep-seated  regional  metamor- 
phism,  and  often  contact  action.  They  are  thus  heavy  minerals  which 
develop  from  the  simpler  constituents  in  the  argillaceous  sediments  in  the 
zone  of  anamorphism,  their  formation  resulting  in  condensation. 

Alterations. — The  only  alteration  of  the  clintonite  group  recorded  is  that 
of  margarite  to  dudleyite.  However,  as  no  definite  formula  for  this  mineral 
is  given,  it  is  not  practicable  to  write  an  equation  representing  the 
transformation. 

While  no  other  alterations  of  the  clintonite  group  are  mentioned,  there 
is  no  doubt  that  in  the  upper  zone  of  metamorphism,  especially  in  the  belt 
of  weathering,  the  chloritoids,  ottrelite,  and  margarite  are  decomposed  into 
simpler  compounds,  as  are  the  other  silicates. 

CHLORITE    GROUP. 
AMESITE,  rORUXDOPHILITE,  ri:u<  n  MUM  I  I  .  i  l.i  \,n  II I  inn  .  AND  PENMNITE. 

The  minerals  of  the  chlorite  group,  according  to  Tschermak,  may  be 
regarded  as  isomorphous  mixtures  of  amesite  (H4Mg2Al2Si09)  and  serpen- 
tine (H4Mg3Si209)  molecules,  although  Clarke  dissents  from  this  conclusion. 
Tschermak  gives  the  range  of  the  various  orthochlorites  as  follows : 

Amesite:  At  to  At4Sp. 

Corundophilite:  At4Sp  to  At7Sp:). 

Prochlorite  (ripidolite) :  At,Sp3  to  At3Sp3. 
Clinochlore:  At3Sp3  to  At  Sp. 

Penninite:  At  Sp  to  At2Sp3. 

These  would  correspond  to  the  following  compositions: 

Amesite: 

H4Mg2Al2Si09  to  H20Mg11Al8Si6045. 

Monoclinic. 

Sp.  gr.  2.71. 

Corundophilite: 

HjoMgnAlgSieOu  to  H40Mg23AlHSi13Ollo. 

Monoclinic. 

Sp.  gr.  2.90. 


346  A  TREATISE  ON  METAMORPHISM. 

Prochlorite: 

H40Mg2SAluSi13090  to  H20Mg12Al6Si,045. 

Monoclinic. 

Sp.  gr.  2.78-2.96. 

dinochlore: 

H20Mg12Al6Si7O,5  to  H8Mg5Al2Si3O18. 

Monoclinic. 

Sp.  gr.  2.65-2.78. 

Penninile: 

H8Mg5Al2Si,018  to  HMMglsAl4Si8045. 
Pseudorhombohedral  and  monoclinic. 
Sp.  gr.  2.60-2.85. 

A  considerable  part  of  the  magnesium,  as  shown  by  the  analyses, 
may  be  replaced  by  iron,  the  analyses  of  corumlophilite  showing  as  high 
as  15  per  cent  of  monoxide  of  iron;  of  prochlorite,  from  15  to  25  per  cent, 
running  even  higher  than  the  magnesia.  The  percentage  of  monoxide  of 
iron  in  clinochlore  and  penninite  is  usually  much  less.  The  alumina  may 
be  replaced  in  part  by  sesquioxide  of  iron,  although  the  proportion  of  this 
replacement  is  not  nearly  so  great  as  that  of  the  magnesia  by  the  iron  mon- 
oxide, the  iron  sesquioxide  generally  not  running  beyond  2  or  3  per  cent. 

It  is  apparent  that  the  specific  gravities  of  the  chlorites  do  not  regularly 
grade  from  lower  to  higher,  as  in  the  feldspars.  Doubtless  such  a  regular 
gradation  would  occur  provided  the  chlorites  were  pure  magnesium  min- 
erals, corresponding  to  the  formulae  above  given.  The  high  specific  gravities 
which  the  intermediate  minerals  in  the  group,  corundophilite  and  prochlo- 
rite, may  have  are  doubtless  explained  by  their  frequent  high  content  of 
iron  monoxide. 

occurrence. — Chlorite  is  the  most  abundant  and  widespread  of  all  the 
secondary  silicates.  As  a  secondary  mineral  it  is  probably  subordinate 
only  to  quartz.  Chlorite  is  nowhere  known  as  a  pyrogenic  constituent  of 
igneous  rock.  It  is  very  abundant  in  many  of  the  altered  igneous  rocks, 
both  plutonic  and  volcanic,  including  lavas  and  tuffs,  being  especially 
abundant  in  the  so-called  green-schists.  Also  it  is  very  abundant  in  many 
amphibolites.  In  the  altered  igneous  rocks  which  are  changed  under  mass- 
static  conditions  it  is  one  of  the  most  abundant  secondary  constituents, 
being  especially  prevalent  in  the  basic  rocks,  such  as  the  greenstones.  It 
is  also  found  in  the  acid  rocks.  Chlorite  occurs  as  a  plentiful  allogenic 
constituent  in  all  kinds  of  mechanical  sedimentary  rocks.  It  develops  as  a 


ALTERATIONS  OF  CHLORITE.  347 

very  abundant  secondary  constituent  in  the  metamorphosed  sedimentary 
rocks,  such  as  slates,  schists,  and  gneisses.  Frequently  chlorite  may  occur 
more  abundantly  adjacent  to  intrusive  rocks  than  elsewhere.  The  most 
characteristic  associated  secondary  minerals  are  epidote,  serpentine,  talc, 
zeolites,  kaolin,  magnesite,  iron  oxides,  aluminum  oxides,  etc. 

In  the  discussion  of  the  individual  minerals  it  has  been  shown  that 
chlorite  is  one  of  the  abundant  derivation  products  of  the  following  minerals : 
Almandite,  augite,  garnet,  hornblende,  iolite,  prehnite,  pyrope,  staurolite, 
tourmaline,  and  vesuvianite.  Being  essentially  a  magnesium-aluminum 
silicate,  it  is  especially  likely  to  be  derived  from  the  heavily  magnesian 
minerals,  of  which  the  olivine,  pyroxene,  amphibole,  mica,  and  garnet 
groups  are  the  more  important.  As  already  seen,  corundophilite  and  pro- 
chlorite  may  contain  a  large  percentage  of  iron  monoxide,  and  therefore 
one  would  naturally  expect  these  chlorites  to  form  from  the  minerals  which 
also  contain  a  large  percentage  of  iron  monoxide,  as,  for  instance,  olivine, 
actinolite,  etc.  In  many  cases  the  mineral  from  which  the  chlorite  is 
derived  does  not  contain  a  sufficient-amount  of  magnesium.  In  such  cases 
this  substance  is  derived  from  adjacent  minerals,  or  is  brought  in  in  solution. 
It  has  been  supposed  in  such  cases  that  the  magnesium  is  transported  as  a 
carbonate.  However,  the  principles  of  its  development  would  be  in  no 
way  changed  if  any  other  salt  of  magnesium,  such  as  magnesium  chloride, 
were  substituted  for  the  carbonate. 

In  the  discussion  of  the  individual  minerals  it  is  shown  that  chlorite 
develops  especially  in'  the  upper  physical-chemical  zone,  and  particularly  in 
the  belt  of  cementation.  Under  conditions  of  quiescence  it  develops  at 
very  considerable  depth ;  but  in  proportion  as  interior  movement  occurs  it  is 
likely  to  develop  in  smaller  quantity  or  not  at  all,  its  place  as  a  metamor- 
phic  mineral  being  taken  by  the  magnesian  mica  biotite. 

Alterations. — The  alterations  of  chlorite,  like  those  of  other  minerals,  are 
largely  dependent  upon  the  zones  or  belts  in  which  the  mineral  is  located. 
The  only  definite  alteration  products  of  chlorite  which  are  recorded  are 
those  which  Tschermak  has  called  enophite  and  berlanite.  The  first  is  said 
by  him  to  be  a  serpentinous  variety  of  chlorite.  No  formula  for  either  has 
been  determined,  and  therefore  it  is  not  possible  to  write  equations  repre- 
senting the  transformation.  Rosenbusch  says  that  the  last  stage  of  the 
alteration  of  chlorite  is  into  an  aggregate  of  limonite,  carbonate,  and  quartz. 
This  degeneration  is  especially  characteristic  of  the  belt  of  weathering.  As 


348  A  TREATISE  ON  METAMORPHISM. 

usual,  no  attempt  is  made  to  write  equations  for  these  degenerative  changes; 
but  if  one  knew  definitely  the  composition  of  the  original  mineral  and  that 
of  the  minerals  which  were  produced  in  a  given  case,  it  would  be  easy  to 
write  equations  for  the  change  and  to  calculate  the  volume  relations. 

While  the  alterations  of  chlorite  in  the  zone  of  anamorphism  are  not 
recorded,  it  is  certain  that  the  chlorite  of  chloritic  rocks  under  the  condi- 
tions of  the  lower  physical-chemical  zone  pass  into  other  constituents,  since 
chlorite  is  almost  always  rare  or  absent  in  both  the  sedimentary  and  the 
igneous  rocks  which  have  recrystallized  in  the  lower  zone  and  have  not 
been  later  affected  by  changes  in  the  upper  zone. 

Therefore  in  the  lower  zone  chlorite  and  some  of  the  material  of  the 
associated  minerals  recombine  and  reproduce  minerals  from  which  chlorite 
was  originally  derived,  or  other  minerals.  There  is  little  doubt  that  chlorite 
furnishes  a  considerable  part  of  the  elements  for  such  minerals  as  the  micas, 
feldspars,  amphiboles,  pyroxenes,  and  even  the  olivines,  which  develop  in 
the  zone  of  anamorphism,  and  also  it  is  probable  that  the  chlorite  furnishes 
a  part  of  the  constituents  for  certain  of  the  heavy  metamorphic  minerals, 
such  as  garnet,  cliutonite,  staurolite,  tourmaline,  etc. 

SERPENTINE-TALC  GROUP. 
I 

SEKPEXTIXE  AND  TALC. 

The  serpentine-talc  group  includes: 

Serpentine: 

H^MgjSijjCV     (A  part  of  the  Mg  may  be  replaced  by  Fe,  and  where  the  amount  of  Fe  is 

considerable  this  mineral  is  called  bastite.) 
Monoclinic. 
Sp.  gr.  2.50-2.65. 

Talc: 

HsMgsSi4012. 

Orthorhombic  or  monoclinic. 

Sp.  gr.  2.7-2.8. 

Serpentine  and  talc,  like  chlorite,  are  both  hydrous  magnesium  silicates. 
Indeed,  as  has  been  pointed  out,  Tschermak  regards  the  serpentine  mole- 
cule with  the  amesite  molecule  (H4Mg2Al2SiO9)  in  variable  proportions  to 
constitute  the  chlorites.  Serpentine  is  more  hydrous  and  more  basic  than 
talc.  Since  the  serpentine  molecule  is  similar  to  some  of  the  chlorites, 
one  would  expect  that  the  occurrence  of  the  two  would  be  very  similar, 
and  such  is  the  fact. 


OCCURRENCE  AND  ALTERATIONS  OF  SERPENTINE.  349 


SERPENTINE. 


occurrence. — Serpentine  occurs  in  substantially  all  the  varieties  of  rocks  in 
which  chlorite  is  found,  but  it  is  most  abundant  as  a  secondary  constituent 
in  the  igneous  rocks  which  are  very  heavily  magnesian,  especially  the 
pyroxenites,  peridotites,  and  similar  rocks.  Locally  it  may  be  so  abundant 
as  a  secondary  constituent  in  rocks  of  this  class,  especially  those  bearing 
olivine,  as  to  form-  serpentine  rocks.  Serpentine  develops  very  abundantly 
in  the  sedimentary  rocks  which  are  rich  in  magnesian  constituents,  both  in 
detrital  material  from  basic  igneous  rocks,  and  in  limestones,  and  in  various 
transition  varieties.  Serpentine  is  a  product  of  the  zone  of  katamorphism, 
including  both  the  belt  of  cementation  and  the  belt  of  weathering. 

As  shown  under  the  discussion  of  the  various  minerals,  it  is  secondary 
to  actinolite,  biotite,  bronzite,  chondrodite,  clinohumite,  diopside,  enstatite, 
hornblende,  humite,  hypersthene,  muscovite,  oliviue,  pyrope,  sahlite,  and 
spinel.  Of  these  the  most  important  is  olivine,  and  of  second  importance 
are  the  pyroxenes  and  amphiboles.  In  many  cases  the  constituents  out  of 
which  serpentine  is  formed  are  derived  not  from  a  single  mineral,  but  from 
various  minerals,  in  which  case  the  serpentine  may  replace  nonmagnesian 
minerals,  as  feldspar,  or  may  form  in  veins. 

Alterations. — Serpentine,  where  long  exposed  to  the  conditions  of  the  belt 
of  weathering,  is  likely  to  break  up  into  various  minerals,  of  which  brucite 
(rhombohedral ;  sp.  gr.  2.39),  magnesite  (rhombohedral ;  sp.  gr.  3.06),  opal 
(amorphous;  sp.  gr.  2.15),  and  quartz  (rhombohedral;  sp.  gr.  2.6535)  are 
the  more  important.  By  hydration  and  loss  of  magnesia  it  passes  into 
webskyite  (amorphous;  sp.  gr.  1.771). 

The  reaction  by  which  serpentine  passes  into  magnesite,  brucite,  and 
quartz  may  be  written  thus: 

(1)  H4Mg3Si2O9+CO2=MgCOa+2Mg(OH).!+2SiOj+k. 

The  increase  in  volume  is  13.02  per  cent.  In  case  opal  or  hydromagnesite 
were  formed  the  increase  in  volume  would  be  somewhat  greater,  and  the 
reaction  would  involve  hydration  as  well  as  carbonation.  It  is  of  course 
possible  that  both  brucite  and  magnesite  may  not  always  be  formed  simul- 
taneously. If  brucite  and  not  magnesite  be  formed  the  equation  is — 

(2)  H4MgsSi209+H2O=3Mg(OH).1+2SiOa+k. 


350  A  TREATISE  ON  METAMORPHISM. 

The  volume  of  the   brucite  and  quartz  is  9.82  per  cent  greater  than  the 
serpentine-     If  magnesite  and  not  brucite  be  formed  the  equation  is — 

(3)  H4Mg3Si2O9+3CO.,=3MgCO3+2SiO2+2H2O-  k. 

The  volume  of  the  magnesite  and  quartz  is  18.84  per  cent  greater  than  that 
of  the  serpentine. 

Brauns"  gives  the  formula  for  the  development  of  webskyite  as  follows: 

(4)  3(H4R,Si209)— RO+12aq.=2(H6R4Si3013+6aq.) 

Where  serpentine  contains  iron  as  a  base,  partly  replacing  the  magne- 
sium, the  iron  is  oxidized  and  may  be  hydrated,  thus  producing  hematite 
or  limonite. 

The  breaking  up  of  serpentine  occurs  especially  in  the  belt  of  weath- 
ering, the  transformation  representing  one  of  the  final  changes  in  the 
degeneration  of  the  silicates.  Alterations  of  serpentine  in  the  zone  of 
anamorphism  are  not  recorded.  But  the  general  absence  of  serpentine  in 
the  schists  and  gneisses  of  sedimentary  origin  profoundly  metamorphosed 
in  the  zone  of  anamorphism  is  conclusive  evidence  that  the  serpentine  which 
once  was  in  these  rocks,  and  the  associated  secondary  minerals,  have 
recombined  to  produce  heavy  minerals  of  the  classes  from  which  serpentine 
and  those  other  secondary  minerals  were  originally  produced.  One  could 
readily  form  equations  for  such  alterations  by  reading  the  equations  by 
which  serpentine  is  formed  from  right  to  left.  (See  Table  C,  pp.  37f>-394.) 


occurrence. — Talc  is  practically  coextensive  in  its  occurrence  with  chlorite 
and  serpentine,  but  in  its  distribution  is  more  nearly  allied  to  serpentine 
than  to  chlorite.  Therefore  it  is  found  in  almost  every  variety  of  rock  long 
subjected  to  alterations  in  the  belt  of  weathering;  but  it  is  especially 
abundant  in  the  heavily  magnesian  rocks.  Steatite,  which  is  nearly  pure 
talc,  is  usually  derived  from  the  pyroxenites  or  peridotites.  However,  talc  is 
so  abundant  in  many  schists  as  to  give  them  the  name  talcose,  or  even  talc- 
schists.  Also,  like  serpentine,  it  occurs  abundantly  in  the  dolomite-bearing 
rocks  and  in  dolomite. 

« Brauns,  R.,  Studien  iiber  den  Palaeopikrit  von  Amelose  bei  Biedenkopf  und  dessen  Umwand- 
lungsprodukte:  Neues  Jahrbucli,  supp.-vol.  5,  Stuttgart,  1887,  p.  322. 


OCCURRENCE  AND  ALTERATION  OF  GLAUCONITE.  351 

Talc  forms  in  the  upper  zone  of  metamorphism.  In  this  respect  it  is 
like  chlorite  and  serpentine.  It  is  especially  likely  to  form  under  condi- 
tions of  weathering.  The  minerals  from  which  talc  is  derived  are  as 
follows:  Actinolite,  andalusite,  anthophyllite,  bronzite,  cyanite,  diopside, 
enstatite,  gehlenite,  hypersthene,  muscovite,  olivine,  phlogopite,  pyrope, 
sahlite,  sca|)olites,  sillimanite,  spinel,  staurolite,  topaz,  and  tremolite.  The 
manner  of  formation  is  given  under  the  various  minerals.  Of  these  minerals 
the  more  important  are  the  uonaluminous  amphiboles  and  pyroxenes,  both 
orthorhombic  and  monocliuic.  It  also  forms  rather  abundantly  from  olivine, 
mica,  and  garnet. 

Alterations. — Alterations  of  talc  are  not  recorded.  It  appears  to  be  one 
of  the  end  products  of  rock  alteration  in  the  belt  of  weathering.  However, 
I  have  no  doubt  that  when  the  talc-bearing  rocks  are  buried  so  deeply  as  to 
pass  into  the  zone  of  anamorphism  and  there  alteration  takes  place,  talc, 
like  chlorite,  serpentine,  and  other  minerals,  is  destroyed,  and  that  from  it 
alone,  or  from  it  and  other  minerals,  the  classes  of  heavy  minerals  from 
which  the  talc  was  originally  produced  are  again  formed. 

GLAUCONITE. 
Glauconite: 

Essentially  a  hydrous  silicate  of  iron  and  potassium.     Definite  formula  unknown.     The 

potassium  ranges  from  1.85  to  6.56  per  cent. 
Amorphous. 
Sp.  gr.  2.2-2.4. 

occurrence. —  Glauconite  occurs  in  sediments  of  many  kinds  and  ages. 
Where  it  is  so  abundant  as  to  give  the  rock  a  green  color  it  is  known  as 
greensand.  Greensands  are  especially  prevalent  in  the  Cretaceous. 

Alterations. — Since  no  definite  formula  for  glauconite  can  be  given  it  is 
impracticable  to  write  equations  representing  the  transformations.  But 
since  glauconite  is  almost,  if  not  quite,  unknown  in  the  schists  and  gneisses 
formed  in  the  zone  of  anamorphism  it  seems  certain  that  under  the  condi- 
tions of  that  zone  the  glaucouite  is  broken  up,  its  constituents  passing  into 
other  minerals. 


352  A  TREATISE  ON  METAMORPHISM. 


KAOLIN    GROUP. 

Kaolinite  is  the  only  important  rock-making  mineral  of  this  group. 

Kaolinite: 

H4  A12  Si2  O9. 
Monoclinio. 
Sp.  gr.  2.6-2.63. 

occurrence. — Kaolinite  is  a  secondary  product  in  all  classes  of  igneous 
rocks  and  occurs  as  an  important  constituent  in  all  sedimentary  rocks  except 
the  pure  sandstones  and  the  pure  limestones.  Kaolinite  and  quartz  are  the 
chief  constituents  of  the  clays,  and  kaolinite  is  a  very  abundant  constituent 
of  muds  and  grits. 

Kaolinite  is  a  product  which  forms  extensively  in  the  zone  of  katamor- 
phism  in  the  belt  of  cementation  and  in  the  belt  of  weathering.  It  is  likely 
to  be  produced  as  a  result  of  the  decomposition  of  any  of  the  aluminous 
minerals.  It  has  been  noted  as  having  been  produced  from  the  following 
minerals:  Andalusite,  anorthoclase,  biotite,  cyanite,  epidote,  leucite,  micro- 
cline,  nephelite,  orthoclase,  plagioclases,  scapolites,  sillimanite,  sodalite, 
topaz,  and  zoisite.  Of  these,  undoubtedly  the  most  important  are  the  feld- 
spars, and  especially  the  acid  feldspars. 

Alterations. — No  alterations  of  kaolinite  are  recorded.  It  is  certain,  how- 
ever, that  where  the  kaolin-bearing  sediments  are  deeply  buried  the  mineral 
becomes  dehydrated,  that  such  bases  as  the  alkalies  and  alkaline  earths  and 
iron  replace  the  hydrogen,  and  that  various  anhydrous  silicates  or  silicates 
low  in  hydrogen  are  produced.  It  is  certain  that  in  the  zone  of  anamor- 
phism the  minerals  which  in  the  upper  physical-chemical  zone  have  broken 
up  into  kaolinite  as  one  of  the  products  may  recombine  to  a  large  extent 
and  reproduce  the  original  minerals. 

SUMMARY    OF   ALTERATION    OF   SILICATES. 

While  the  important  groups  of  the  rock-forming  silicates  have  been 
treated  separately,  it  may  be  well  in  closing  the  section  to  class  together 
the  groups  of  the  original  minerals  which  have  a  somewhat  similar  chemical 
composition  and  therefore  alter  into  somewhat  similar  products. 

These  classes  are  called  by  the  petrographers  (1)  the  feldspathoid 
class,  (2)  the  transition  class,  and  (3)  the  ferromagnesian  class.  The  felds- 
pathoid class  includes  the  feldspar,  nephelite,  sodalite,  leucite,  and 
wernerite  groups.  The  only  rock-forming  minerals  belonging  to  the 


SUMMARY  OF  ALTERATION  OF  SILICATES.  353 

transition  class  are  those  of  the  inuscovite  group.  The  ferromagnesian 
class  includes  the  pyroxene,  araphibole,  chrysolite,  biotite-phlogopite,  and 
clintonite  groups. 

(1)  In  the  upper  physical-chemical  zone,  that  of  katamorphism,  the 
more  common  alteration  products  of  the  feldspathoid  class  are  the  kaolins, 
the  zeolites,  the  epidotes  (including  both  zoisite  and  epidote  proper),  diaspore, 
gibbsite,  and  quartz.     Probably  all  of  these  minerals  form  in  both  belts  of 
the  zone,  but  the  development  of  the  zeolites  and  the  epidotes  is  more 
characteristic  of  the  belt  of  cementation  than  of  the  belt  of  weathering. 
Indeed,  as  pointed  out  under  these  minerals,  under  long-continued  condi- 
tions of  the  belt  of  weathering  these  minerals  break  up  into  carbonates 
of  the    alkalies  and  alkaline    earths,   hydrous  and  anhydrous,  oxides  of 
aluminum  and  iron,  quartz,  and,  probably,  also  kaolin.     In  the  zone  ot 
anamorphism   the   more   common  alteration  products  of  the  feldspathoid 
class  are  muscovite  (damourite)  and  scapolite.     The  nephelite,  sodalite,  and 
leucite  groups  alter  into  the  feldspars. 

(2)  In   the  zone  of  katamorphism  muscovite  alters  into  serpentine, 
talc,   and  vermiculite   (hydromuscovite).     In  the  belt  of  weathering  the 
serpentine   and  vermiculite  may  break  up  into  simpler  compounds  of  the 
same  character  as  those  which  form  from  the  zeolites  and  epidotes. 

In  the  lower  physical-chemical  zone,  that  of  anamorphism,  muscovite 
is  one  of  the  minerals  commonly  produced,  and  therefore  does  not  usually 
alter.  But  by  profound  and  deep-seated  metamorphism,  the  material  of 
muscovite  may  pass  into  the  heavy  ferromagnesian  minerals,  such  as  garnet, 
staurolite,  etc.  % 

(3)  The   ferromagnesian    silicates    may   be    divided   into   two   great 
divisions — those  which  are  nonaluminous,  and  those  which  are  aluminous. 
In  the  zone  of  katamorphism  the  most  common  alteration  products  of  the 
nonaluminous    ferromagnesian    silicates   are    talc    and    serpentine.     The 
nonaluminous   pyroxenes    and  amphiboles  ordinarily  pass  into   talc;  the 
chrysolites  ordinarily  pass  -into  serpentine.     The  transformations  in  these 
directions  are  explained  by  the  fact  that  the  pyroxenes,  amphiboles,  and  talc 
are  metasilicates,  while  the  oliviues  and  serpentines  are  orthosilicates.     The 
metasilicates  naturally  pass  into  metasilicates,  and  the  orthosilicates  into 
orthosilicates.     In  the  zone  of  katamorphism  the  heavily  aluminous  ferro- 
magnesian silicates  alter  into  chlorites  and  epidotes.       The  pyroxenes  and 
amphiboles  which  are  not  heavily  aluminous  frequently  split  up  into  a  com- 


354  A  TREATISE  ON  METAMORPHISM. 

bination  of  chlorite  and  talc,  the  aluminous  part  of  the  original  molecule 
going  to  the  chlorite  and  the  nonaluminous  part  into  the  talc.  The 
development  of  epidote  is  largely  if  not  wholly  confined  to  the  belt  of 
cementation.  But  chlorite  apparently  forms  both  in  the  belt  of  cementa- 
tion and  in  the  belt  of  weathering,  especially  where  there  is  abundant 
vegetation.  Under  extreme  and  long-continued  conditions  of  the  belt  of 
weathering  the  serpentines,  chlorites,  and  epidotes  are  likely  to  further 
.  degenerate,  breaking  up  into  carbonates  of  the  alkalies  and  alkaline  earths, 
anhydrous  or  hydrous  oxides  of  aluminum  and  iron,  quartz,  and  kaolin. 
Or  in  the  belt  of  weathering  these  end  products  may  directly  develop  from 
the  metasilicates  without  the  serpentines,  chlorites,  and  epidotes  as  inter- 
mediate products.  In  the  zone  of  anamorphism  the  pyroxenes  change  to 
amphiboles;  the  pyroxenes  and  amphiboles  both  alter  to  biotite;  the 
olivines  change  to  the  amphiboles,  anthophyllite,  tremolite,  and  actinolite. 
The  biotite  group  does  not  ordinarily  alter.  But  by  profound  metamor- 
phism  the  material  of  the  biotites,  amphiboles,  pyroxenes,  etc.,  may  pass  into 
the  still  heavier  class  of  minerals  represented  by  the  garnets,  staurolites,  etc. 


THE   TITANATES. 
TITANITK   AND   PKROV8KITE. 


As  rock-making  constituents  only  two  titanates  of  importance,  titanite 
and  perovskite,  are  here  treated,  ilmenite  being  given  under  the  oxides. 


TManite: 

CaTiSiO5. 
Monoclinic. 
Sp.  gr.  3.4-3.56. 

Peronkiti:- 
CaTiOs. 

Isometric  or  pseudoisometric. 
Sp.  gr.  4.017-4.039. 


occurrence. — Titanite  occurs  as  a  rather  subordinate  but  widespread  min- 
eral as  an  original  pyrogenic  constituent  of  igneous  rocks,  and  also  in  the 
schists  and  gneisses.  So  far  as  observed,  titanite  as  a  secondary  constituent 
is  derived  from  ilmenite  and  rutile.  These  alterations  are  discussed  under 
those  minerals. 


ALTERATIONS  OF  TITANITE.  355 

Alterations.  —  Titaiiite  alters  into  rutile  (tetragonal;  sp.  gr.  4.18-4.25), 
octahedrire  (tetragonal;  sp.  gr.  3.82-3.95),  and  perovskite  (isometric  or 
pseudoisometric  ;  sp.  gr.  4.017-4.039). 

Rutile  and  octahedrite  may  be  supposed  to  be  produced  by  the  follow- 
ing reaction: 

(1  )  CaTi8iO5+CO.,==TiO2+CaCO3+SiO,+k. 

The  expansion  of  volume  is   39.22  per  cent  for  rutile,  provided  all  of  the 
compounds  separate  as  solids,  and  42.07  for  octahedrite. 

Perovskite  may  be  supposed  to  be  produced  by  the  simple  breaking 
up  of  titanite,  according  to  the  reaction: 


(2)     CaTiSiO5=CaTi<  )s 

The  expansion  of  volume  is  0.14  per  cent  provided  the  silica  also  separates 
as  a  solid. 

Information  as  to  the  natural  conditions  under  which  these  changes 
take  place  is  not  obtainable  from  the  papers  giving  the  above  minerals  as 
secondary  to  titanite.  From  the  character  of  the  first  change  one  would 
expect,  however,  that  it  would  occur  in  the  zone  of  katamorphism,  and 
especially  in  the  belt  of  weathering'.  Under  such  conditions  there  would  be 
a  reason  for  the  change,  for  there  carbonation  of  the  silicates,  with  libera- 
tion of  heat  and  with  expansion  of  volume,  is  the  rule.  As  so  frequently 
indicated  before,  the  freed  silica  may  be  taken  into  solution,  and  if  this 
occurs  the  volume  is  decreased.  Under  what  conditions  the  second  reaction 
is  likely  to  have  taken  place  I  can  only  conjecture  from  its  nature.  I 
should  expect  it  to  occur  in  the  zone  of  katamorphism. 


PEROVSKITE. 


occurrence. — Perovskite  occurs  as  an  original  constituent  in  eruptive 
rocks,  and  also  in  the  metamorphic  rocks,  such  as  the  schists  and  gneisses. 
As  a  secondary  mineral  it  has  been  observed  as  a  product  secondary  to  titan- 
ite. It  may  be  suspected  that  in  the  schists  and  gneisses  it  forms  in  the 
zone  of  anamorphism  by  the  union  of  rutile  and  octahedrite  or  brookite, 
with  calcium  carbonate;  but  this  is  a  pure  conjecture,  as  the  details  of  its 
formation  are  not  found  in  literature. 

Alterations. — The  mineral  does  not  readily  alter  into  other  compounds, 
although  it  has  been  observed  to  alter  into  an  undetermined  substance,  and 
it  is  said  to  alter  into  ilmenite  (hexagonal-rhombohedral ;  sp.  gr.  4.75). 


356  A  TREATISE  ON  METAMORPHISM. 

THE    PHOSPHATES. 
APATITE. 

The  only  important  rock-making  mineral  among  the  phosphates  is 
apatite. 

Apatite: 

CaF.Ca4P3O12,  or  CaCl.Ca4P3O12,  or  a  mixture  of  the  two. 

Hexagonal. 

Sp.  gr.  3.17-3.23. 

occurrence. — Apatite  is  one  of  the  most  widespread,  if  not  the  most  wide- 
spread, of  all  the  subordinate  constituents  of  rocks.  It  is  a  common,  if  not 
an  almost  universal,  constituent  of  the  plutonic  rocks,  occurs  almost  as 
broadly  in  the  volcanic  rocks,  and  is  found  in  many  varieties  of  unaltered 
or  little  altered,  sedimentary  rocks,  such  as  limestones,  shales,  sandstones, 
etc.;  and,  finally,  it  is  almost  everywhere  found  in  the  metamorphosed 
igneous  and  sedimentary  rocks. 

Alterations. — The  only  alteration  which  is  recorded  for  apatite  is  to  osteo- 
lite,  which  is  reported  as  having  the  same  composition  as  apatite,  except 
that  there  has  been  a  loss  of  part  or  all  of  the  fluorine  or  chlorine. 
I  It  is  certain,  however,  that  in  the  belt  of  weathering  of  the  zone  of 
anamorphism  apatite  is  slowly  dissolved.  This  is  shown  by  comparative 
analyses  of  the  weathered  with  the  unweathered  varieties  of  the  same 
rock.  This  fact  has  been  frequently  noted  in  reference  to  the  iron  ores, 
because  here  the  presence  or  absence  of  phosphorus  is  of  such  great  impor- 
tance. It  may  be  stated  that  in  the  iron  ores  it  is  the  general  rule  that  those 
parts  of  deposits  which  have  been  long  subjected  to  weathering  bear  a 
smaller  proportion  of  apatite  than  the  continuations  of  these  same  deposits 
in  the  belt  of  cementation. 

The  depletion  of  the  surface;  rocks  in  apatite  would  seem  to  furnish  an 
adequate  source  for  the  apatite  in  veins,  this  mineral  being  taken  into 
solution  near  the  surface  and  redeposited  deeper  down,  thus  being  trans- 
ported from  the  belt  of  weathering  to  the  belt  of  cementation. 


ANHYDRITE  AND  GYPSUM,  357 

THE   SULPHATES. 
ANHYDRITE   AND    GYPSUM. 

The  only  important  rock-making  sulphates  are  anhydrite  and  gypsum. 

Anhydrite; 

CaSOt 

Orthorhombic. 
Sp.  gr.  2.899-2.985. 

Gypsum: 

CaSO4 .2H20 

Monoclinic. 

Sp.  gr.  2.314-2.328. 


ANHYDRITE. 


occurrence. — As  explained  below,  the  main  source  of  anhydrite  is  by  the 
alteration  of  gypsum  in  the  zone  of  anamorphism.  Although  I  do  not 
know  the  facts,  I  conjecture  that  the  anhydrite  deposits  of  Switzerland 
have  had  such  a  history. 

Alterations. — 'Phe  chief  alteration  of  anhydrite  is  to  gypsum  (monoclinic; 
sp.  gr.  2.314-2.328).  The  reaction  is: 

(1)     CaSO4+2H2O=CaS01.2H2O+k 

The  increase  in  volume  is  60.30  per  cent.  This  alteration  is  one  which 
takes  place  ill  the  zone  of  katamorphism.  An  interesting  case  is  that  of 
Bex,  Switzerland,  where  the  transformation  from  anhydrite  to  gypsum  has 
taken  place  completely  to  a  depth  of  from  18  to  30  meters,  and  where 
below  this  depth  the  material  is  anhydrite.  The  change  of  anhydrite  to 
gypsum  is  with  liberation  of  heat,  expansion  of  volume,  hydration,  lowering 
of  specific  gravity,  and  lessening  of  symmetry,- and  thus  stands  as  a  rare 
example  of  all  the  tendencies  of  the  upper  physical-chemical  zone. 


occurrence. — The  most  important  source  of  gypsum  is  as  a  chemical 
precipitate,  especially  in  salt  lakes  having  no  outlets.  It  therefore  natu- 
rally occurs  in  association  with  halite,  calcite,  and  mechanical  detritus. 
Gypsum  also  is  produced  in  a  subordinate  way  through  fumarole  action. 
The  calcium  sulphate  for  the  gypsum  in  either  case  is  produced  by  the 
reaction  of  sulphuric  acid  or  sulphates  upon  calcium-bearing  salts.  Com- 


358  A  TREATISE  ON  METAMO11PHISM. 

monly  the  sulphate  is  formed  by  the  oxidation  of  a  sulphide.  A  common 
method  is  the  production  of  iron  sulphate  by  oxidation  of  pyrite,  marcasite, 
or  pyrrhotite,  which  reacts  upon  calcium  carbonate,  thus  producing  gypsum. 
The  reaction  is — • 

CaCOs+FeSO4+2H2O=CaSO4.2H2O+FeCO3+k. 

The  development  of  gypsum  by  this  method  is  illustrated  at  many  mines. 
Finally  gypsum  may  be  formed  by  the  hydration  of  anhydrite.  All 
these  methods  of  formation  of  gypsum  are  characteristic  of  the  zone  of 
katamorphism,  and  especially  of  the  belt  of  weathering. 

Alterations. — An  important  alteration  of  gypsum  is  to  anhydrite  (ortho- 
rhombic,  sp.  gr.  2.899-2.985).  -  The  reaction  is — 

(1)  CaSO4.2H2O=CaS04+2H.,O-K. 

The  decrease  in  volume  is  37.62  per  cent.  The  other  important  alteration 
of  gypsum  is  into  calcite  (rhombohedral,  sp.  gr.  2.713-2.714).  The 
reaction  is — 

(2)  CaSO4.2H2O-CO2=CaCO,+H2SO4+H2O+k. 

The  H,S()4  produced  may  simultaneously  react  upon  some  other  compound. 
The  decrease  in  volume  is  50.29  per  cent. 

Unless  beds  of  gypsum  have  been  deeply  buried  the  alteration  to 
anhydrite  has  not  extensively  occurred.  It  is  a  reaction  of  diminution  of 
volume,  absorption  of  heat,  dehydration,  increase  in  specific  gravity,  and 
increase  in  symmetry,  and  therefore  is  one  of  the  very  rare  cases  which 
illustrate  all  the  tendencies  of  the  lower  physical-chemical  zone.  The 
change  of  gypsum  to  calcite  is  a  reaction  with  liberation  of  heat  and  con- 
densation of  volume.  The  change  takes  place  near  the  surface,  especially 
in  the  belt  of  weathering,  where  carbon  dioxide  is  abundant,  and  may  also 
occur  in  the  lower  zone.  It  therefore  stands,  in  its  physical-chemical 
relations,  in  the  same  position  as  dolomitization.  (See  p.  240.) 


MINERALS.  359 

SECTION  4.— GENERAL  STATEMENTS. 

PHYSICAL-CHEMICAL   FACTORS    ON   WHICH    NATURE   OF   ALTERATIONS 

DEPENDS. 

As  inferences  from  the  foregoing'  treatment  it  may  be  said  that  the  more 
important  physical-chemical  factors  on  which  the  alteration  of  an  individual 
mineral  depends  are  (1)  the  chemical  composition  of  the  mineral,  (2)  the 
chemical  composition  of  the  adjacent  minerals,  (3)  the  chemical  composi- 
tion of  the  circulating  solutions,  (4)  the  specific  gravity,  (5)  the  symmetry, 
(6)  the  heat  effect  of  the  reaction,  and  (7)  the  pressure  and  volume. 

CHEMICAL   COMPOSITION. 

Certain  chemical  compounds  are  stable  under  a  great  variety  of 
conditions;  others  are  stable  only  under  certain  definite  conditions;  and  thus 
the  chemical  composition  influences  the  stability  of  minerals.  As  an  illus- 
tration of  minerals  which  have  stability  under  widely  varying  conditions 
may  be  mentioned  quartz,  which  forms  alike  from  a  magma  and  from  water 
solutions,  and  also  at  the  surface  and  at  great  depth.  Nephelite  and  soda- 
lite  are  examples  of  minerals  which  can  exist  only  under  a  comparatively 
narrow  range  of  conditions. 

CHEMICAL   COMPOSITION   OF   ADJACENT   MINERALS. 

It  has  been  seen,  that  mineral  particles  may  react  upon  one  another, 
either  through  the  medium  of  contained  solutions  or  by  direct  rearrange- 
ment under  the  influence  of  pressure.  Therefore,  it  is  clear  that  the  nature 
of  a  mineral  which  is  mainly  secondary  to  another  mineral  is  influenced 
by  the  chemical  compositions  of  adjacent  minerals. 

CHEMICAL   COMPOSITION   OF   CIRCULATING   SOLUTIONS. 

It  has  already  been  shown  that  the  secondary  minerals  are  dependent 
not  only  upon  the  adjacent  minerals,  but  upon  the  material  carried  by  the 
underground  solutions.  The  amount  of  such  material  is  dependent  upon 
the  vigor  of  the  circulation.  As  explained  on  pages  507-518,  655-656, 
764-766,  the  material  added  or  abstracted  may  be  great  in  the  zone  of 
katamorphism,  but  is  usually  rather  limited  in  amount  in  the  zone  of 
anamorphism. 


360  A  TREATISE  ON  METAMOKPHISM. 


SPECIFIC   GRAVITY. 


Apparently  high  specific  gravity  is  favorable  to  stability.  This  is  what 
one  would  expect,  for  high  specific  gravity  involves  a  comparatively  close 
arrangement  of  the  atoms.  Where  the  atoms  are  near  together  the 
molecular  attraction  is  great,  and  in  order  to  break  up  the  combination 
this  force  must  be  overcome.  This  principle  is  illustrated  by  dimorphous 
and  trimorphous  compounds.  Diamond  (av.  sp.  gr.  3.52)  is  more  stable  than 
graphite  (av.  sp.  gr.  2.16),  and  graphite  is  more  stable  than  carbon  (sp.  gr. 
1.9,  charcoal).  Pyrite  (av.  sp.  gr.  5.025)  is  more  stable  than  marcasite  (av. 
sp.  gr.  4.870).  Cyanite  (av.  sp.  gr.  3.615)  is  more  stable  than  sillimanite 
(av.  sp.  gr.  3.235),  and  sillimanite  is  more  stable  than  andalusite  (av.  sp.  gr. 
3.18).  Quartz  (av.  sp.  gr.  2.6535)  is  more  stable  than  tridymite  (av.  sp.  gr. 
2.305).  This  last  instance  well  illustrates  the  principle;  for  the  symmetry 
of  quartz  and  tridymite  is  the  same,  and  this  variable  factor  included  in 
the  previous  illustrations  is  excluded.  The  same  is  true  of  andalusite 
and  sillimanite  of  the  aluminum-silicate  series.  As  pointed  out  on  page 
112,  the  more  condensed  a  compound,  or,  in  other  words,  the  higher  the 
specific  gravity,  the  less  energy  is  potentialized.  In  the  change  from  a 
lower  to  a  higher  specific  gravity  energy  is  liberated.  In  this  we  have  the 
physical  explanation  of  the  greater  stability  of  minerals  of  high  specific 
gravity.  To  form  minerals  of  higher  specific  gravity  from  those  of  lower 
specific  gravity  releases  energy.  To  reproduce  minerals  of  lower  specific 
gravity  from  those  of  higher  specific  gravity  requires  the  expenditure  of 
energy.  An  exception  to  the  above  rule  as  to  increase  of  stability  with 
increase  of  specific  gravity  is  furnished  by  calcite  and  aragonite.  Calcite 
(av.  sp.  gr.  2.7135)  is  more  stable  than  aragonite  (av.  sp.  gr.  2.94),  but  in 
this  case  the  factor  of  symmetry  enters,  which  is  discussed  under  the  next 
heading. 

SYMMETRY. 

Apparently  the  greater  the  symmetry  the  more  stable  is  the  mineral 
likely  to  be.  This  principle  is  illustrated  by  substances  which  are  dimor- 
phous or  trimorphous. 

Pyrite  (isometric)  is  more  stable  than  marcasite  (orthorhombic). 
Diamond  (isometric)  is  more  stable  than  graphite  (hexagonal),  and  this 
is  more  stable  than  amorphous  carbon.  Kelvin  suggests0  that  soft 

a  Lord  Kelvin,  Popular  lectures  and  addresses,  Macmillan  &  Co.,  London,  1894,  vol.  2,  p.  428. 


SPECIFIC  GRAVITY  AND  SYMMETRY.  361 

phosphorus  as  compared  with  red  phosphorus,  and  prismatic  sulphur  as 
compared  with  octahedral  sulphur,  contain  potential  energy.  When  the 
change  from  the  first  to  the  second  takes  place,  energy  is  evolved,  and 
consequently  the  second  form  is  more  stable.  These  changes  are  in  the 
direction  of  higher  symmetry,  and  Kelvin's  argument  applies  equally  well 
to  all  the  changes  in  which  minerals  pass  from  a  lower  to  a  higher  degree  of 
symmetry.  To  reproduce  minerals  of  lower  symmetry  would  require  the 
expenditure  of  energy.  Therefore  we  have  an  energy  cause  why  minerals 
with  high  symmetry  are  more  stable.  They  contain  less  potential  energy. 
Their  formation  is  under  the  apparent  law  of  the  universe  of  dissipation  of 
energy. 


SPECIFIC   GRAVITY    AND   SYMMETRY. 


Where  specific  gravity  and  symmetry  work  together,  as  in  a  number 
of  the  illustrations  mentioned,  there  seem  to  be  no  exceptions  to  the  rule 
of  increase  of  stability  with  increase  of  specific  gravity  and  increase  in 
symmetry. 

But  in  those  instances  in  which  the  specific  gravity  and  symmetry  are 
opposed  to  each  other  it  can  not  be  predicted  which  will  be  the  dominant 
factor.  For  instance,  calcium  carbonate  crystallizes  as  calcite  (hexagonal- 
rhombohedral;  sp.  gr.  2.7135)  and  aragonite  (orthorhombic ;  sp.  gr.  2.94). 
The  former  is  the  more  stable.  In  this  case  it  seems  that  symmetry  is  the 
dominant  factor.  In  the  aluminum  silicate  which  crystallizes  as  andalusite 
(orthorhombic;  sp.  gr.  3.18),  sillimanite  (orthorhombic;  sp.  gr.  3.235),  and 
cyanite  (triclinic;  sp.  gr.  3.615),  the  latter  is  the  most  stable.  In  this  case 
it  appears  that  the  specific  gravity  is  the  determining  factor. 

It  is  believed  that  when  the  energy  relations  of  these  changes  become 
known  it  will  be  found  that  in  each  of  these  cases  "the  more  stable  molecules 
contain  less  potential  energy.  If  this  be  true,  calcite,  considering  both  its 
specific  gravity  and  its  symmetry,  contains  less  energy  than  aragonite,  and 
cyanite  less  than  andalusite  or  sillimanite.  If  this  conjecture  be  true,  all 
compounds  are  subject  to  a 'common  law.  That  mineral  forming  from  a 
compound  is  most  stable  in  which  the  minimum  energy  is  contained. 

The  relations  of  symmetry  and  specific  gravity  raise  some  very 
interesting  questions  as  to  the  arrangement  of  the  molecules  in  minerals. 
Pressure  undoubtedly  tends  to  produce  the  most  compact  arrangement. 


362  A  TREATISE  ON  METAMORPHISM. 

(See  pp.  182-186.)  According  to  Slichter,  the  most  compact  possible 
arrangement  of  spherical  molecules  is  that  which  gives  a  rhpmbohedral  unit 
having  face  angles  equal  to  60°  and  120°. "  One  might  therefore  conclude, 
other  things  being  equal,  that  minerals  having  hexagonal  crystallization  would 
be  those  which  have  the  closest  arrangement  of  molecules  and  therefore  the 
highest  specific  gravity;  but  plainly  there  are  other  factors  entering  into 
the  problem,  for,  as  already  pointed  out,  aragonite  (orthorhombic)  has  a 
higher  specific  gravity  than  calcite  (-hexagonal),  and  cyanite  (tri clinic) 
has  a  higher  specific  gravity  than  sillimanite  and  andalusite  (orthorhombic). 
On  the  other  hand,  diamond  (isometric)  has  a  higher  specific  gravity  than 
graphite  (hexagonal).  This  is  an  especially  interesting  case,  since  the 
cubical  arrangement  of  molecules,  the  one  ordinarily  appealed  to  to 
explain  isometric  symmetry,  is  the  most  open  of  all  possible  arrangements. 
From  the  foregoing  it  appears  perfectly  clear  that  besides  the  manner  of  the 
arrangement  of  the  molecular  particles  the  distance  of  the  molecules  from 
one  another  enters  as  a  very  important  factor.  Also  the  shape  of  the 
molecules,  the  closeness  of  the  arrangement  of  their  atoms,  and  the  com- 
plexity of  the  molecules  themselves  doubtless  enter  as  important  factors 
into  the  density  of  minerals. 

HEAT   REACTIONS. 

Other  things  being  equal,  within  the  lithosphere  reactions  take  place 
which  give  the  greatest  liberation  of  heat.  This  law  is  best  illustrated 
at  or  near  the  surface,  where  the  reactions  usually  occur  in  accordance 
with  it.  •  The  reactions  of  oxidation,  hydration,  and  carbonation  are  there- 
fore dominant  However,  the  law  of  reactions  with  liberation  of  heat 
becomes  less  and  less  able  to  control  as  the  pressure  becomes  con- 
siderable. Where  the  pressure  is  great,  as  noted  under  the  next  heading, 
it  determines  the  reaction  without  respect  to  whether  heat  is  absorbed  or 
liberated,  and  in  many  cases  the  reactions  take  place  with  the  absorption 
of  heat,  so  far  as  the  chemical  factors  are  concerned.  If  all  the  physical 
factors  also  were  included,  all  reactions  would  take  place  with  the  dissipa- 
tion of  energy.  (See  p.  57.) 

"felichter,  C.  S.,  Theoretical  investigation  of  the  motion  of  ground  waters:  Nineteenth  Ann.  Kept. 
U.  S.  Geol.  Survey,  pt.  2,  1899,  pp.  306-310. 


PHYSICAL-CHEMICAL  FACTORS  363 


PRESSURE    AND   VOLUME. 


Pressure  lessens  the  volume  and  therefore  tends  to  preserve  and  to  pro- 
duce minerals  which  have  a  high  specific  gravity.  Where  the  pressure 
is  small  this  factor  is  relatively  inefficient  and  consequently  other  factors 
usually  control,  and  many  minerals  of  low  specific  gravity  form.  But  even 
where  the  pressure  is  small  it  is  not  unimportant,  at  least  retarding  reactions 
which  would  otherwise  occur.  This  is  illustrated  by  a  partially  altered 
rock  described  by  Merrill,0  which  seemed  solid  when  confined  by  the 
surrounding  rock,  but  which  when  brought  to  the  surface  from  a  depth  of 
a  few  feet,  and  thus  freed  from  pressure,  rapidly  decomposed  and  disinte- 
grated. Where  the  pressure  is  great  it  is  likely  to  be  a  determinative 
force,  Controlling  the  reactions.  At  the  depths  of  the  zone  of  anamorphism 
the  uniform  production  of  anhydrous  or  slightly  hydrous  minerals  of  higher 
average  specific  gravity  than  those  formed  in  the  zone  of  katamorphism  is 
clear  evidence  of  the  dominance  of  pressure. 

In  this  connection  it  will  be  well  to  mention  the  mineral  groups,  with 
their  specific  gravities,  which  are  more  characteristic  of  the  zones  of 
katamorphism  and  anamorphism. 

The  characteristic  products  of  the  zone  of  katamorphism  are:  Of  the 
oxides,  (1)  those  of  silicon,  opal,  chalcedony,  and  quartz  (sp.  gr  2.1  to 
2.654);  (2)  those  of  iron,  including  the  hydrous  and  anhydrous  ferric- 
oxides  (sp.  gr.  3.80  to  5.225);  (3)  the  hydrous  aluminum  oxides,  gibbsite 
and  diaspore  (sp.  gr.  2.35  and  3.40);  of  the  carbonates,  calcite  and  dolo- 
mite (sp.  gr.  2.7135  and  2.85);  of  the  silicates,  (1)  the  epidote-zoisite  group 
(sp.  gr.  3.25  to  4.20);  (2)  zeolite  group  (sp.  gr.  2.04  to  2.40);  (3)  chlorite 
group  (sp.  gr.  2.60  to  2.96);  (4)  serpentine-talc  group  (sp.  gr.  2.50  to  2.80); 
and  kaolin  group  (sp.  gr.  2.6  to  2.63).  (See  pp.  519.-520,  621-627.) 

The  characteristic  important  mineral  groups  formed  in  the  zone  of 
anamorphism  are  as  follows:  Of  the  sulphides,  pyrite,  and  pyrrhotite  (sp. 
gr.  5.025  and  4.61);  of  the  oxides,  (1)  those  of  silicon,  chert,  chalcedony, 
and  quartz  (sp.  gr.  2.6  to  2.654);  (2)  those  of  iron,  hematite,  magnetite,  and 
ilmenite  (sp.  gr.  5.225,  5.174,  and  4.75);  (3)  those  of  aluminum,  corundum 
(sp.  gr.  4.025);  (4)  those  of  titanium,  rutile,  octahedrite,  and  brookite  (sp. 
gr.  4.215,  3.885,  and  3.975);  of  the  silicates,  (1)  the  feldspar  group  (sp.  gr. 

"Merrill,  George  P.,  Rocks,  rock  weathering,  and  soils,  Macmillan  Co.,   New  York,  1897,  pp. 
252-253. 


364  A  TREATISE  ON  METAMORPHISM. 

2.54  to  2.76);  (2)  pyroxene  group  (sp.  gr.  2.68  to  3.58):  (3)  amphibole 
group  (sp.  gr.  2.9  to  3.713);  (4)  garnet  group  (sp.  gr.  3.41  to  4.30);  (5)  chrys- 
olite group  (sp.  gr.  3.2  to  4.1);  (6)  scapolite  group  (sp.  gr.  2.566  to  2.74); 
(7)  aluminum  silicate  group  (sp.  gr.  3.16  to  3.67);  (8)  humite  group  (sp.  gr. 
3.1  to  3.2);  (9)  tourmaline  (sp.  gr.  3.09);  (10)  staurolite  (sp.  gr.  3.71);  (11) 
mica  group  (sp.  gr.  2.7  to  3.1);  (12)  clintonite  group  (sp.  gr.  2.9  to  3.57). 

The  average  specific  gravity  of  the  mineral  groups  above  mentioned 
as  products  of  the  zone  of  katamorphism  is  2.948.  The  average  specific 
gravity  of  the  mineral  groups  of  the  zone  of  anamorphism  is  3.488.  It 
thus  appears  that  the  average  specific  gravity  of  the  minerals  which  develop 
in  the  zone  of  anamorphism  is  18  per  cent  greater  than  that  of  the 
minerals  in  the  zone  of  katamorphism.  This  comparison  is  of  course  very 
roughly  approximate,  since  the  various  minerals  are  not  present  in  equal 
quantities.  Probably  the  percentage  is  too  great,  since  the  heavy  sulphides 
and  the  very  heavy  silicates  are  given  equal  weight  with  the  abundant  but 
lighter  quartz,  feldspars,  pyroxenes,  amphiboles,  and  micas.  The  com- 
parison, however,  shows  beyond  question  that  a  given  mass  of  material 
occupies  much  less  space  in  the  lower  physical-chemical  zone  than  in  the 
upper  physical-chemical  zone. 

It  is  shown  under  the  next  heading  that  many  of  the  reactions  written 
for  the  minerals  of  the  zone  of  katamorphism  may  be  read  in  reverse  order 
when  the  resultant  minerals  are  buried  so  deep  as  to  be  in  the  zone  of 
anamorphism.  The  lighter  minerals  characteristic  of  the  zone  of  kata- 
morphism reunite  to  produce  heavier  minerals  of  the  zone  of  anamorphism, 
such  as  the  feldspars,  the  micas,  the  pyroxenes,  the  amphiboles,  the  chryso- 
lites, andalusite,  etc.  .  Furthermore,  where  the  pressure  is  great  enough 
these  minerals  rearrange  themselves  again  in  whole  or  in  part  so  as  to 
produce  still  heavier  minerals,  such  as  garnet,  staurolite,  tourmaline, 
sillimanite,  cyanite,  etc.  This  great  change  takes  place  within  the  narrow 
range  of  less  than  10,000  meters. 

Since  in  this  mere  outer  film  of  the  earth  a  great  diminution  in  the 
volume  of  the  minerals  has  taken  place,  it  is  thought  to  be  highly  probable 
that,  even  if  the  average  chemical  composition  of  the  interior  of  the  earth 
be  supposed  to  be  the  same  as  the  crust,  the  pressure  is  such  that  the  min- 
erals may  further  rearrange  themselves  into  still  more  compact  products, 
thus  probably  producing  minerals  of  a  different  kind  and  higher  specific 


PRESSURE  AND  VOLUME.  365 

gravity  than  any  with  which  we  are  acquainted.  Indeed,  the  interior 
pressures  increase  so  rapidly  with  depth  that  rearrangement  might,  occur 
again  and  again.  Therefore,  even  if  the  average  chemical  composition  be 
the  same  deep  within  the  earth  as  at  the  surface,  in  the  centrosphere,  in 
consequence  of  high  pressure,  there  may  be  a  set  of  silicate  minerals  which 
have  as  high  a  specific  gravity  as  the  average  density  of  the  earth,  viz, 
5.67.  If  the  accepted  theory  as  to  the  distance  between  molecules  be  cor- 
rect, viz,  that  molecules  of  ordinary  liquids  at  the  surface  of  the  earth  do  not 
occupy  more  than  one-third  of  the  total  volume,"  there  is  ample  room 
between  them  for  the  condensed  rearrangement  suggested.  From  the  fore- 
going it  appears  that  we  do  not  necessarily  appeal  to  a  great  preponderance 
of  heavy  metals  deep  within  the  earth  to  explain  its  average  high  specific 
gravity.  It  may  be  very  largely  explained  by  the  condensation  of  the 
material  due  to  pressure.  If,  as  suggested  by  Chamberlin,  the  average 
specific  gravity  of  the  material  of  the  earth  be  that  of  meteoric  falls,  the 
average  change  in  specific  gravity  would  be  from  3.69  to  5.67  as  a  result  of 
pressure.  The  great  increase  in  the  average  specific  gravity  of  minerals 
with  increase  of  pressure  in  the  crust  of  the  earth  would  seem  to  make  the 
estimate  of  the  change  in  average  specific  gravity  of  the  minerals  from  3.69 
to  5.6.7,  as  a  result  of  the  very  great  pressures  deep  within  the  earth,  a  very 
modest  one. 

\Yhile  I  have  no  doubt  that  the  condensation  of  the  earth  material  into 
heavier  compounds  as  a  result  of  pressure  is  a  partial  explanation  of  the 
high  specific  gravity  of  the  earth,  I  by  no  means  urge  this  as  the  sole  cause. 
Indeed,  it  is  probable  that  the  segregation  of  heavy  material  toward  the 
center  and  lighter  material  toward  the  surface  has  steadily  continued 
throughout  geological  time,  and  therefore  the  difference  in  composition  is  a 
very  important  factor  in  the  difference  in  density  at  the  surface  and  the 
center.  But  I  do  not  venture  even  a  guess  as  to  the  relative  importance  of 
the  two  factors  of  condensed  compounds  and  segregation  of  material  in 
explaining  the  increase  in  density  of  the  material  of  the  earth  with  increase 
of  depth. 

oNernst,  W.,  Theoretical  chemistry,  trans,  by  C.  S.  Palmer,  Macmillan  &  Co.,  London,  1895, 
p.  196. 


366  A  TREATISE  ON  METAMORPHISM. 


REVERSIBLE    REACTIONS. 


On  the  foregoing  pages  numerous  reactions  have  been  written  by 
which  the  minerals  characteristic  of  the  zone  of  katamorphism  are  pro- 
duced; very  few  reactions  have  been  written  by  which  the  minerals  of  the 
zone  of  anamorphism  are  reproduced.  It  is  certain  that  when  the  minerals 
formed  in  the  belts  of  weathering1  and  cementation  are  altered  under  the 
conditions  of  the  zone  of  anamorphism  the  minerals  characteristic  of  that 
zone  develop;  therefore  it  is  believed  that  many  of  the  reactions  for  the 
development  of  the  minerals  of  the  zone  of  katamorphism  are  reversible. 
To  illustrate,  in  the  zone  of  katamorphism  olivine  may  alter  into  the  min- 
erals serpentine,  magnetite,  magnesite,  and  quartz,  according  to  the  fol- 
lowing equation : 

3MgsFeSi2O8+3CO2+4H20+O=2H4Mg3Si2O9+FeA+3MgCOs4-2SiO2+k. 

It  is  believed  that  when  these  four  minerals  are  brought  together  in  proper 
proportions  under  favorable  conditions  in  the  deep-seated  zone  the  reverse 
reaction  occurs,  and  that  the  equation  may  be  read  from  right  to  left 
instead  of  left  to  right,  thus  reproducing  the  olivine. 

The  above  illustration  is  chosen  because  the  change  from  left  to  rio-lit 
involves  carbonation,  desilication,  hydration,  and  oxidation;  and  the  change 
from  right  to  left  involves  silication,  decarbonation,  dehydration,  and  deoxi- 
dation.  Of  course,  where  deoxidation  takes  place  in  the  zone  of  anamor- 
phism some  reducing  agent  must  be  present  to  utilize  the  abstracted  oxygen. 
The  principle  of  the  reversibility  of  the  reactions  in  the  two  opposing  zones 
is  actually  illustrated  in  a  few  cases  where  the  products  of  the  zone  of  kata- 
morphism have  been  observed  to  alter  in  the  zone  of  anamorphism.  For 
instance,  it  is  recorded  (p.  261)  that  aualcite  is  derived  from  albite  according 
to  the  following  equation: 

2NaAlSi809+2H20=Na2Al2SitOI2-2H20+2Si02+k; 

whereas  we  find  (p.  334)  that  aualcite  alters  to  albite  by  the  reaction: 

Na2Al2Si4OI2-2H2O+2SiO2=2NaAlSi308+2H2O-k. 

Ill  other  words,  the  reaction  is  exactly  reversible;  for  while  the  k  is  plus 
in  the  first  equation  and  minus  in  the  second,. it  is  on  opposite  sides  in  the 
two  equations.  The  feldspars  alter  into  many  zeolites,  and  a  number  of 


REVERSIBLE  REACTIONS.  367 

the  zeolites  alter  into  the  various  feldspars.  The  above  reaction  chances 
to  be  the  only  one  given  for  these  groups  which  is  exactly  reversed.  This 
is  a  consequence  of  the  fact  that  reactions  are  written  only  for  recorded 
alterations.  There  can  be  no  doubt  that  practically  all  the  equations 
representing  the  recorded  alterations  of  the  feldspars  (pp.  261-263)  into 
the  zeolites,  and  all  the  reactions  representing  the  recorded  alterations  of 
the  zeolites  (pp.  333-334)  into  the  feldspars,  are  reversible.  For  instance, 
we  have  anorthite  altering  into  gismondite  as  follows  (p.  262): 

3CaAl2Si2O8+12H2O=Ca.,A]6Si6O24.12H2O+k. 

Can  one  doubt  that  if  gismondite  passes  into  the  zone  of  anamorphism 
dehydration  may  take  place  and  anorthite  be  reproduced! 

Another  line  of  evidence  pointing  to  the  reversibility  of  the  reactions 
in  the  two  zones  is  the  frequent  recorded  association  of  corundum  with 
diaspore  and  gibbsile,  the  latter  minerals  being  secondary  to  the  corundum. 
Can  it  be  doubted  that  these  hydrates  may  be  dehydrated  in  the  zone  of 
anamorphism  and  reproduce  corundum!  Of '  course  this  particular  change 
may  not  occur  alone.  At  the  same  time  the  dehydration  takes  place  the 
alumina  may  unite  with  silica  and  form  andalusite,  sillimanite,  or  cyanite, 
or  the  alumina  may  enter  into  some  other  silicate. 

Bearing  in  the  same  direction  are  the  experiments  made  by  Daubre"e 
upon  serpentine."  It  is  well  known  that  both  enstatite  and  olivine  alter 
into  serpentine.  Daubree  found  that  by  the  fusion  of  serpentine  it  split  up 
into  enstatite  and  olivine,  according  to  the  following  equation: 

H4Mg2Si2O9+Heat=MgSiO3+MgSiO4+2H20+k. 

Finally,  my  chief  reason,  in  addition  to  those  already  given,  for  belief 
in  the  reversibility  of  the  reactions  in  the  two  zones  lies  in  the  actual 
compositions  of  the  unmetamorphosed  sediments  and  their  metamorphosed 
equivalents.  The  unmetamorphosed  pelites  are  composed  largely  of  the 
lighter  hydrous  minerals  of  the  belt  of  weathering  and  the  belt  of  cementa- 
tion. It  is  true  that  with  these,  as  already  explained,  there  are  also 
considerable,  or  even  dominant,  quantities  of  residual  undecomposed 
anhydrous  minerals;  but  it  is  certain  that  the  metamorphosed  equivalents 
of  these  pelites  contain  none  of  the  minerals  which  are  characteristic  of 

a  Daubree,  A.,  Experiences  synthe'tiques  relatives  aux  me^orites:  Comptes  rendus  des  stances 
de  1'academie  des  sciences,  vol.  62,  Paris,  1866,  p.  661. 


368  A  TREATISE  OX  M  KTAMORPHISM. 

the  belts  of  weathering  and  cementation,  and  the  only  possible  conclusion 
is  that  these  minerals  have  recombined  and  reproduced  the  heavier  minerals 
of  the  lower  physical-chemical  zone.  That  this  is  so  is  shown  by  the  fact 
that,  barring  the  water  and  the  carbon  dioxide  which  are  liberated  in  the 
process  of  alteration,  the  average  chemical  compositions  of  the  unaltered 
pelites  and  their  metamorphosed  equivalents  are  nearly  the  same. 

While  it  is  held  that  the  reactions  are  reversible,  it  is  not  supposed 
that  this  is  often  exactly  the  case  for  a  given  rock.  In  order  that  this 
should  even  approximately  take  place,  it  would  be  necessary  that  there  be 
no  change  of  average  composition  in  the  zone  of  katamorphism,  and  this  is 
never  the  case.  The  minerals  formed  in  the  zone  of  anamorphism  depend 
not  only  upon  the  minerals  of  the  zone  of  katamorphism  present,  but  upon 
their  proportion  and  many  other  factors.  What  is  meant  by  the  reversi- 
bility of  the  reactions  is  that,  when  compounds  produced  in  the  zone  of 
katamorphism  from  a  given  mineral  are  together  in  proper  proportions 
and  conditions  in  the  zone  of  anamorphism,  the  original  mineral  may  be 
reproduced. 

If  this  law  of  the  reversibility  of  reactions  in  the  two  zones  be  true, 
the  question  naturally  arises  why  so  few  of  the  reversing  reactions  in 
the  zone  of  anamorphism  have  been  recorded.  The  answer  lies  in  the 
difference  in  the  readiness  with  which  observations  may  be  made  in  the 
two  zones.  The  reactions  of  the  belts  of  weathering  and  cementation  of 
the  zone  of  katamorphism  have  been  more  fully  described,  because  they 
are  constantly  taking  place  at  or  near  the  surface  under  conditions  of 
ready  observation.  Many  of  the  reverse  reactions  have  not  been  fully 
described,  because  they  occur  at  depth,  and  because  in  areas  of  strong 
metamorphic  action  they  have  been  complete.  Usually  gradation  from 
practically  complete  reactions  to  very  incomplete  reactions  in  the  zone  of 
anamorphism  is  comparatively  rapid.  But  notwithstanding  the  very  imper- 
fect observations  of  the  zone  of  anamorphism,  the  general  reversibility  of 
the  reactions  in  the  two  zones  seems  as  certain  as  if  it  were  established  by 
observation,  and  it  is  believed  that  it  will  be  established  by  observation. 

If  the  conclusions  of  the  foregoing  paragraphs  be  correct  it  is  evident 
that  there  is  an  almost  entirely  neglected  field  of  observation  in  metamor- 
phism — that  by  which  the  minerals  of  the  zone  of  anamorphism  are  produced 
from  the  minerals  of  the  zone  of  katamorphism. 


TABLES.  369 

For  the  reasons  given  above,  I  conclude:  It  is  believed  that  most  of 
the  equations  which  represent  the  reactions  in  the  zone  of  katamorphism  are 
reversible  in  the  zone  of  anamorphism;  and  so  far  as  there  is  expansion  of 
volume  and  liberation  of  heat  in  the  upper  zone,  just  so  far  is  there  condensation 
of  volume  and  absorption  of  heat  in  the  lower  zone. 

SECTION  5.     TABLES. 

In  order  to  present  compactly  the  essential  facts  as  to  the  alterations 
of  each  mineral,  a  set  of  tables  is  here  given. 

Table  A  gives  the  mineral  sources  of  each  of  the  minerals. 

Table  B  gives  the  minerals  to  which  each  mineral  alters. 

Table  C  gives  the  equations  representing  the  alterations  of  each  of  the 
minerals  into  other  minerals  and  shows  the  volume  changes. 

Table  D  classifies  the  alterations  of  the  minerals  under  processes  and 
gives  their  various  combinations,  with  volume  changes. 

TABLE  A. — Sources  of  minerals. 

Acmite  is  derived  from arfvedsonite. 

Actinolite  is  derived  from ankerite,  bronzite,  hypersthene,  olivine,  parankerite, 

sahlite. 

Albite  is  derived  from : analcite,  heulandite,  laumontite,  plagioclases  (with  or- 

thoclase),  sodalite,  spodumene,  stilbite. 

Allophane  is  derived  from anorthoclase,  microcline,  orthoclase. 

Amesite  is  derived  from pyrope. 

Analcite  is  derived  from laumontite,  leucite,  nephelite,  plagioclases,  sodalite. 

Anhydrite  is  derived  from gypsum. 

Anthophyllite  is  derived  from bronzite,  hypersthene,  olivine. 

Aphrosiderite  is  derived  from garnet.  . 

Augite  is  derived  from hornblende. 

Bastite  is  derived  from actinolite,  anthophyllite,  bronzite,  cummingtonite, 

hyperethene,  sahlite. 

Berlanite  is  derived  from chlorite. 

Beta-spodnmene  is  derived  from spodumene. 

Biotite  is  derived  from anorthoclase,  augite,  hornblende,  microcline,  ortho- 
clase, seapolites. 

Biotite-chlorite  is  derived biotite. 

Brucite  is  derived  from chondrodite,  clinohumite,  humite,  serpentine. 

Breunerite  is  derived  from olivine. 

Calcite  is  derived  from actinolite,  ankerite,  anthophyllite,  aragonite,  augite, 

diopside,  dolomite,  epidote,  fluorite,  garnet,  grossu- 
larite,  gypsum,  hauynite,  hornblende,  noselite,  paran- 
kerite, sahlite,  seapolites,  tremolite,  zoisite. 

Chabazite  is  derived  from hauynite,  noselite,  plagioclases. 

Chalcedony  is  derived  from augite,  sahlite. 

Chlorite  is  derived  from almandite,  augite,  biotite,  garnet,  hornblende,  iolite, 

phlogopite,  prehnite,  pyrope,  staurolite,  tourmaline, 
vesuvianite. 

MON    XLVII — 04 24 


370  A  TREATISE  ON  METAMORPHISM. 

TABLE  A. — Sources  qf, minerals — Continued. 

Cimolite  is  derived  from anorthoelase,  microcline,  orthoclase. 

Chlorophyllite  is  derived  from iolite. 

Chromite  is  derived  from olivine. 

Clinochlore  is  derived  from biotite  (with  phlogopite). 

Corundum  is  derived  from diaspore,  gibbsite. 

Cyanite  is  derived  from andalusite,  corundum,  diaspore,  gibbsite. 

Cymatolite  is  derived  from spodumene. 

Damourite  is  derived  from andalusite,  corundum,  cyanite,  microcline,  orthoclase, 

sillimanite,  staurolite,  topaz. 

Diaspore  is  derived  from biotite,  corundum,  garnet  (conjectural ) ,  gibbsite,  haiiyn- 

ite,  muscovite,  nephelite,  noselite,  phlogopite,  scapo- 
lites,  sodalite. 

Diopside  in  derived  from dolomite. 

Dolomite  is  derived  from ankerite,  calcite,  parankerite. 

Dudley ite  is  derived  from margarite. 

Enophite  is  derived  from chlorite. 

Enstatite  is  derived  from .pyrope. 

Epidote  is  derived  from anorthoclase,  augite,  biotite,  garnet,  hornblende,  micro- 
cline, orthoclase,  plagioclases,  scapolites. 

Epistilbite  is  derived  from plagioclases. 

Eucryptite  is  derived  from spodumene. 

Fassaite  is  derived  from gehli-nite. 

Garnet  is  derived  from vesuvianite. 

Gibbsite  is  derived  from anorthoclase,  andalusite,  biotite,  cancrinite,  corundum, 

cyanite,  epidote,  garnet  (conjectural),  haiiynite,  mi- 
crocline, muscovite,  nephelite,  noselite,  orthoclase, 
phlogopite,  plagioclases,  pyrope,  scapolites,  silliman- 
ite, sodalite,  topaz,  tourmaline,  zoisite. 

Gismondite  is  derived  from plagioclases. 

Grossularite  is  derived  from gehlenite. 

Griinerite  is  derived  from siderite. 

Gypsum  is  derived  from anhydrite. 

Halloysite  is  derived  from anorthoclase,  microcline,  orthoclase. 

Hematite  is  derived  from actinolite,  ankerite,   anthophyllite,   biotite,    bronzite, 

garnet,  greenalite,  griinerite,  hornblende,  hyper- 
sthene,  ilmenite,  limonite,  magnetite,  marcasite,  oli- 
vine, parankerite,  pyrite,  serpentine,  siderite. 

Hercynite  is  derived  from olivine. 

_Heulandite  is  derived  from plagioclases. 

Hornblende  is  derived  from augite,  garnet. 

I  lydrobiotite  is  derived  from biotite. 

I 1  yd  romagnesite  is  derived  from brucite. 

HydromUflCOVite  is  derived  from nephelite,  scapolites,  sodalite. 

Hydronephelite  is  derived  from nephelite,  sodalite. 

Hydrophlogopite  is  derived  from phlogopite. 

Hydrotalcite  is  derived  from olivine. 

Hypersthene  is  derived  from almandite,  biotite,  garnet. 

Ilmenite  is  derived  from perovskite,  rutile. 

Kaolin  is  derived  from andalusite,  anorthoclase,  biotite,  cyanite,  epidote, 

garnet  (conjectural),  leucite,  microcline,  nephelite, 
orthoclase,  the  plagioclases,  the  scapolites,  silliman- 
ite, sodalite,  topaz,  and  zoisite. 

Laumontite  is  derived  from anorthite. 


TABLES.  371 

TABLE  A. — Sources  of  minerals — Continued. 

Lepidomelane  is  derived  from arfvedsonite. 

Limonite  is  derived  from actinolite,  ankerite,  anthophyllite,  arfvedsonite,  biotite, 

bronzite,  chlorites,  epidote,  garnet,  greenalite,  griin- 
erite,  hematite,  hornblende,  hypersthene,  ilmenite, 
magnetite,  marcasite,  olivine,  parankerite,  pyrite, 
pyrrhotite,  serpentine,  siderite. 

Magneeite  is  derived  from garnet,  olivine,  pyrope,  serpentine. 

Magnetite  is  derived  from actinolite,  ankerite,  arfvedsonite,  augite,  biotite,  bronz- 
ite, diopside,  garnet,  greenalite,  griinerite,  hematite, 
hornblende,  hypersthene,  iluienite,  marcasite,  olivine, 
parankerite,  pyrite,  pyrrhotite,  sahlite,  siderite. 

Malacon  (hydrous  zircon)  is  derived  from zircon. 

Marcasite  is  derived  from hematite. 

Margarite  is  derived  from corundum,  diaspore,  gibbsite. 

Meionite  is  derived  fr< >ni grossularite. 

Mesolite  is  derived  from plagioclases. 

Mica  is  derived  from spinel,  tourmaline,  vesuvianite. 

Microcline  is  derived  from spodunu  ne. 

Muscovite  is  derived  from anorthoclase,   diaspore,   gibbsite,     leucite,   microcline, 

iiepbeliti',    orthoclase,    plagioclase   and    orthoclase, 
scapolites,  sodalite,  spodumene. 
See  also  Damourite. 

Natrolite  is  derived  from apatite,  chabazite,  hauynite,  nephelite,  noselite,  plagio- 

clases,  sodalite. 

Nephelite  is  derived  from leucite,  sodalite  ( conjectural). 

Newtonite  is  derived  from anorthosite,  microcline,  orthoclase. 

Octahedrite  is  derived  from ilmenite,  titanite. 

Opal  is  derived  from olivine,  serpentine. 

Orthoclase  is  derived  from analcite,  heulandite,  leucite,  laumontite,  atilbite. 

Osteolite  is  derived  from apatite. 

Paragonite  is  derived  from : anorthoclase,  muscovite,  plagioclases. 

Peetolite  is  derived  from apophyllite. 

Penninite  is  derived  from biotite  (with  phlogopite). 

Perovskite  is  derived  from titanite. 

Phillipsite  is  derived  from plagioclases. 

Phlogopite-chlorite  is  derived  from phlogopite. 

Finite  is  derived  from iolite. 

Prehnite  is  derived  from analcite,  laumontite,  mesolite,   natrolite,  plagioclases, 

scolecite. 

Pyrite  is  derived  from marcasite,  pyrrhotite. 

Pyrophyllite  is  derived  from anorthoclase,  microcline,  orthoclase. 

Quartz  is  derived  from actinolite,  anorthite,  anorthoclase,  anthophyllite,  augite, 

biotite,  bronzite,  chalcedony,  chlorites,  cummington- 
ite,  diopside,  enstatite,  epidote,  garnet,  grossularitc, 
hornblende,  hypersthene,  microcline,  olivine,  opal, 
orthoclase,  'plagioclases,  prehnite,  pyrope,  sahlite, 
scapolites,  serpentine,  tridymite,  zoisite. 

Rutile  is  derived  from brookite,  ilmenite,  octahedrite,  titanite. 

Sahlite  is  derived  from ankerite,  parankerite. 

Scapolites  are  derived  from plagioclases. 

Scolecite  is  derived  from plagioclases. 

Serpentine  is  derived  from actinolite,  biotite,  bronzite,  chondrodite,  clinohumite, 

diopside,  enstatite,  hornblende,  humite,  hypersthene, 
muscovite,  olivine,  pyrope,  sahlite,  spinel. 


372  A  TREATISE  ON  METAMORPHISM. 

TABLK  A. — Sources  of  minerals — Continued. 

Siderite  is  derived  from art" vedsonite,  garnet,  hematite,  hornblende,  limonite, 

magnetite,  olivine. 

Sillimanite  is  derived  from andalusite,  biotite,  corundum,  cyanite,  diaspore,  gibbsite. 

Smaragdite  is  derived  from diallage. 

Sodalite  is  derived  from nephelite. 

Spinel  is  derived  from almandite,  biotite,  corundum,  diaspore,  garnet,  gibbsite, 

olivine,  pyrope. 

Steatite  is  derived  from andalusite,  cyanite,  muscovite,  sillimanite,  topaz,  tour- 
maline. 

Stilbite  is  derived  from haiiynite,  noselite,  plagioclases. 

Talc  is  derived  from actinolite,  andalusite,  anthophyllite,  bronzite,  cyanite, 

diopside,  enstatite,  gehlenite,  hypersthene,  musco- 
vite, olivine,  phlogopite,  pyrope,  sahlite,  scapolites, 
sillimanite,  spinel,  staurolite,  topaz,  tremolite. 

Titanite  is  derived  from ilmenite,  rutile. 

Thomsonite  is  derived  from nephelite,  plagioclases,  sodalite. 

Tremolite  is  derived  from diopside,  dolomite,  olivine. 

Vermiculite  is  derived  from muscovite. 

Webskyite  is  derived  from serpentine. 

Wollastonite  is  derived  from calcite,  dolomite. 

Zoisite  is  derived  from corundum,  diaspore,  gibbsite,  grossularite,  plagioclases. 

TABLE  B. — Alteration  products  of  minerals. 

Actinolite  alters  to bastite,  calcite,  hematite,  limonite,  magnetite,  serpen- 
tine, talc,  quartz. 

Albite  alters  to analcite,  gibbsite,  kaolin,  niarialite,  natrolite,  quartz. 

Almandite  alters  to chlorite,  hypersthene,  spinel. 

Analcite  alters  to albite,  orthoclase,  prehnite. 

Andalusite  alters  to cyanite,  kaolin,  gibbsite,  muscovite  (damourite) ,  silli- 
manite, talc  (steatite). 

Anhydrite  alters  to gypsum. 

Ankerite  alters  to actinolite,  calcite,  dolomite,  hematite,  limonite,  mag- 
netite, sahlite. 

Anorthite  alters  to gibbsite,  gismondite,  kaolin,  laumontite,  meionite, 

prehnite,  quartz,  scolecite,  thomsonite,  zoisite. 

Anorthoclase  alters  to allophane,  biotite,  cimolite,  damourite,  epidote,  gibbs- 
ite, halloysite,  kaolin,  muscovite,  newtonite,  para- 
gonite,  pyrophylite,  quartz. 

Anthophyllite  alters  to bastite,  calcite,  hematite,  limonite,  quartz,  talc. 

Apatite  alters  to osteolite. 

Apophy llite  alters  to pectolite. 

Aragonite  alters  to calcite. 

Arf vedsonite  alters  to  .  acmite,  lepidomelane,  limonite,  magnetite. 

Augite  alters  to biotite,  calcite,  chalcedony,  chlorite,  epidote,  horn- 

blende, magnetite,  quartz. 

Beta-spodumene  alters  to albite,  eucryptite,  muscovite. 

Biotite  alters  to  ...  chlorite,  diaspore,  epidote,  gibbsite,  hematite,  hydro- 

biotite,  hypersthene,  kaolin,  limonite,  magnetite, 
quartz,  serpentine,  sillimanite,  spinel. 

Biotite  (with  phlogopite)  alters  to penninite,  clinochlore. 

Bronzite  alters  to actinolite,  anthophyllite,  bastite,  hematite,  limonite, 

magnetite,  quartz,  serpentine,  talc. 

Brookite  alters  to rutile. 

Brucite  alters  to hydromagnesite. 


TABLES.  373 

TABLE  B. — Alteration  products  of  minerals- — Continued. 

Calcite  alters  to dolomite,  wollastonite. 

Cancrinite  alters  to calcite,  gibbsite,  natrolite. 

Chabazite  alters  to natrolite. 

Chlorite  alters  to berlanite,  enophite,  limonite,  quartz. 

Chondrodite  alters  to brucite,  serpentine. 

Clinohumite  alters  to brucite,  serpentine. 

Corundum  alters  to diaspore,  cyanite,  gibbsite,    margarite,  muscovite  (da- 

mourite),  sillimanite,  spinel,  zoisite. 

Cummingtonite  alters  to bastite,  quartz. 

Cyanite  alters  to kaolin,  gibbsite,  muscovite  (damourite),  talc  (steatite). 

Diallage  alters  to calcite,  chlorite,   epidote,  feldspar,   magnetite,  quartz, 

smaragdite. 
Diaspore  alters  to corundum,  cyanite,  margarite,  muscovite,  sillimanite, 

spinel,  zoisite. 

Diopside  alters  to calcite,  magnetite,  quartz,  serpentine,  talc,  tremolite. 

Dolomite  alters  to calcite,  diopside,  tremolite,  wollastonite. 

Enstatite  alters  to quartz,  serpentine,  talc. 

Epidote  alters  to calcite,  gibbsite,  kaolin,  limonite,  quartz. 

Fluorite  alters  to calcite. 

Garnet  alters  to aphrosiderite,  calcite,  chlorite,  diaspore  (conjectural), 

epidote,  gibbsite  (conjectural),  hematite,  hornblende, 

hypersthene,  iron  oxide,  kaolin  (conjectural),  limon- 
ite, magnesite,  magnetite,  quartz,  giderite. 

Gehlenite  alters  to fassaite,  grossularite,  talc. 

Gibbsite  alters  to corundum,    cyanite,   diaspore,    margarite,    muscovite, 

sillimanite,  spinel,  zoisite. 

Grossularits  alters  to calcite,  meionite,  quartz,  zoisite. 

Griinerite  alters  to hematite,  limonite,  magnetite. 

Gypsum  alters  to anhydrite,  calcite. 

Haiiynite  alters  to calcite,  chabazite,  diaspore,  gibbsite,  natrolite,  stilbite. 

Hematite  alters  to limonite,  magnetite,  marcasite,  pyrite,  siderite. 

Heulandite  alters  to albite,  orthoclase. 

Hornblende  alters  to augite,    biotite,    calcite,    chlorite,    epidote,   hematite, 

magnetite,  quartz,  serpentine,  siderite. 

Humite  alters  to brucite,  serpentine. 

Hypersthene  alters  to actinolite,  anthophyllite,  bastite,   hematite,   limonite, 

magnetite,  quartz,  serpentine,  talc. 
Ilmenite  alters  to hematite,     limonite,     magnetite,     octahedrite,    rutile, 

titanite. 

lolite  alters  to chlorite,  chlorophyllite,  pinite. 

Laumontite  alters  to albite,  analcite,  orthoclase,  prehnite. 

Leucite  alters  to analcite,  kaolinite,  muscovite,  nephelite,  orthoclase. 

Limonite  alters  to hematite,  siderite. 

Magnetite  alters  to hematite,  limonite,  siderite. 

Marcasite  alters  to , hematite,  limonite,  magnetite,  pyrite. 

Margarite  alters  to dudleyite. 

Marialite  alters  to biotite,  kaolin,  muscovite,  quartz,  talc. 

Meionite  alters  to biotite,  calcite,  epidote,  gibbsite,  kaolin,  muscovite. 

Mesolite  alters  to prehnite. 

Microcline  alters  to allophane,  biotite,  cimolite,  damourite,  epidote,  gibbsite, 

halloysite,   kaolin,   muscovite,  newtonite,  pyrophyl- 

lite,  quartz. 
Muscovite  alters  to diaspore,  gibbsite,  paragonite,  serpentine,  talc  (steatite), 

vermiculite. 


374  A  TREATISE  ON  METAMORPHISM. 

TABLE  B. — Alteration  products  of  minerals — Continued. 

Natrolite  alters  to prehnite. 

Nephelite  alters  to albite  (conjectural),  analcite,  diaspore,  gibbsite,  hydro- 

muscovite  (pinite),  hydronephelite,  kaolin,  inusco- 
vite,  natrolite,  sodalite,  thomsonite. 

Noselite  alters  to calcite,  chabazite,  diaspore,  gibbsite,  natrolite,  stilbite. 

Octahedrite  alters  to rutile. 

Olivine  alters  to actiuolite,  anthophyllite,  breunnerite,  chromite,  hema- 
tite, hercynite,  hydrotalcite,  limonite,  magnes-ite,  mag- 
netite, opal,  quartz,  serpentine,  siderite,  spinel,  tremo- 
lite. 

Opal  alters  to chalcedony,  chert,  quartz. 

Orthoclase  alters  to allophane,  biotite,  cimolite,  damourite,  epidote,  gibbsite, 

halloysite,  kaolin,  muscovite,  newtonite,  pyrophyl- 
lite,  quartz. 

Parankerite  alters  to actinolite,  calcite,  dolomite,  hematite,  limonite,  magne- 
tite, sahlite. 

Perovskite  alters  to ilmenite. 

Phlogopite  alters  to chlorite,  diaspore,  gibbsite,  hydrophlogopite,  talc. 

Phlogopite  (with  biotite)  alters  to clinochlore,  penninite. 

Plagioclases  alter  to analcite,  chabazite,  epidote,  epistilbite,  gibbsite,  gis- 

mondite,  heulandite,  kaolin,  laumontite,  mesolite, 
natrolite,  paragonite,  phillipsite,  prehnite,  quartz, 
scapolites,  scolecite,  stilbite,  thomsonite,  zoisite. 

Prehnite  alters  to chlorite,  quartz. 

Pyrite  alters  to hematite,  limonite,  magnetite. 

Pyrope  alters  to amesite,  chlorite,  enstatite,  gibbsite,  magnesite,  quartz, 

serpentine,  spinel,  talc. 

Pyrrhotite  alters  to limonite  magnetite,  pyrite. 

Rutile  alters  to hematite,  ilmenite,  titanite.  , 

Sahlite  alters  to actinolite,  bastite,  calcite,  chalcedony,  magnetite,  quartz, 

serpentine,  talc. 

Sea  polite  alters  to biotite,  calcite,  diaspore,  epidote,  gibbsite,  hydro- 

muscovite  (pinite),  kaolin,  muscovite,  quartz,  talc. 

Scolecite  alters  to prehnite. 

Serpentine  alters  to brucite,  hematite,  limonite,  magnesite,  opal,  quartz, 

webskyite. 

Siderite  alters  to griinerite,  hematite,  limonite,  magnetite. 

Sillimanite  alters  to cyanite,  kaolin,  gibbsite,  muscovite  (damourite),  talc. 

Sodalite  alters  to albite  (conjectural),  analcite,  diaspore,  gibbsite,  hydro- 

muscovite  (pinite),  hydronephelite,  kaolin,  musco- 
vite, natrolite,  nephelite  (conjectural),  thomsonite. 

Spinel  alters  to mica,  serpentine,  talc. 

Spodumene  alters  to albite,  beta-spodumene,  cymatolite,  eucryplitite,  micro- 

cline,  muscovite. 

Staurolite  alters  to chlorite,  gi  I  ilisitc.  magnetite,  muscovite  (damourite),  talc. 

Stilbite  alters  to albite,  orthoclase. 

Titanite  alters  to calcite,  octahedrite,  perovskite,  quartz,  rutile. 

Topaz  alters  to gibbsite,  kaolin,  muscovite,  talc  (steatite). 

Tourmaline  alters  to biotite,  chlorite,  gibbsite,  mica,  steatite. 

Tremolite  alters  to calcite,  talc. 

Vesuvianite  alters  to chlorites,  garnets,  micas. 

Zircon  alters  to hydrous  zircon  (malacon). 

Zoisite  alters  to calcite,  gibbsite,  kaolin,  quartz. 


TABLES. 


375 


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Chemical  reactions. 

4NaAlSi3O8+3CaAl8SisO8J-21H.,O+2CO.,-3(H4CaAl2Si6O18.3H2O)  +2Na«CO3-t  4A1(OH)3  
4NaAlSi3O8+3CaAl2Si2O,+24H2O+2CO.!-Ca3Ale(Si3O8)6.18H2O+2Na.CO3+4Al(OH)3  
6NaAlSi3O8+6CaAl2Si2O8+3CO»+45H2O=  1 

h2Al(OH)8+CaCO...... 

,—  ' 

x(NttAlSi3O8)+2(HCa2Al3Si3Oi3)  +  H2KAl3Si3O12+2SiO2) 
2(2NaA18i3OB.KAlSi3O8)+6H»O+3CO=-3H4Al2Si.O»+12SiO2+KsCO3+2Na.,CO8  

2(2NaAlSi3O8.KAlSi3O8)+9H2O+3CO2-6Al(OH)3+18SiO2+K.CO3+2Na2CO3  
/ 
2NaAlSi,O8.  K  AlSi.iO8+6Al(OH)3-KH2Al3Si3O12+2NaH»Al3Si3O,2+6H2O  

2NaAlSi3O,,.KAlSi3O8+MgCO3+FeCO3+5Al(OH)3=  1 

HKMgFeAl2Si3O12+2NaH2Al.^i3O12+5H2O+2C02| 
2(3NaAlSi3O8.KAlSi3O8)+2Fe2O3+8CaCO3+2H2O=  1 

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3CaAl8SioO8+9H2O  +  COo-2CaAl.,Si3Oio.3H2O+2Al(OH)3+CaCO8  

4NaA18i3O8+3CaAl2Si2O8+13H2O+CO2=2(H8Na2CaAl4SieOs4.H2O)+6SiO2- 

4CaAl2Si.O8+8H2O-2HoCa2Al^i3Ol2+4Al(OH  )3+2SiO2  
dr.a  Al-St~n,  J-SH«n~TT«Pn.  Al-Si.O  0-TT.  AUSi«n« 

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4CaAUSi2Og+7H2O+Fe2O3-H2Ca4Al4Fe^i6O2li+4Al(OH)3+2Si02  

3Ca  A  USi.O8  +  CaCO,  -  Ciu  Al6Si9O«6+  CO2  .  .  . 

378 


A  TREATISE  ON  METAMORPHISM. 


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2[CasMgFeSi40,:.(MgFe 
Ca,Mg,Fe£i,,Osl.(Mg 
2  [Ca6Mg4Fe2Si,2030.  Mg4F 
2[H,K6Mg,,Ff4AlaFc3 
(Not  formulated)  .  .  . 

(Not  formulated)  .  .  . 

6KHMg«AI2Si3O12+18H2C 
2HKMg.Al2Si3012+7H20 
2HKMg.Al2Sl3012+4MgC 
6HjK2Mg3FeAl4Si0O24+2C 

I 


g 

s 

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TABLES. 


379 


-I 


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4H5Ca1oAl12Fe3Si15O05+30SiO2+12AlO(OH)+60MgCO3+15K.,CO3J 
30KHMgoAl2Si3Oi.+6Fe2O3+40CaCO3+29COs=  1 

E035X+O!H+!O!SnVP+!lO*!S3JESH=sOa+KO'!S>tVajIE»KsM"H 
HO)OIVr,l+i;O!S08+59OOIISE3d!1IVllIB05HJ-+[(HO)l''51(V:!Sr-|VfaK-rll](H: 
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380 


A  TREATISE  ON  METAMORPHISM. 


TABLE  C.  —  Chemical  reactions  and  volume  changes  —  Continued. 

Volume 
change. 

fc      1     •< 

1      + 

:::::::::::::::+         +    +    + 

Products. 

S          : 

I        !   !   i   I 

a              .... 

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'•     '•    -    «    1    1    S    1    §     i 

N|  11  !  tllli 
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|_|_J_|_J.  5  I  5  I  5  ! 

Dolomite  . 

rtn 

J   •§   j    t 

p         ^    c 

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Source. 

s 

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Calcite  
...do.. 

(Calcite,  quartz  
Icalcite  
Chabazite  
Chalcedonv  and  ch 

s                                           ¥                          :    2     :    2     : 

I                        I                    S  ?  s  1   1 

*      1               I            HUM 

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Chemical  reactions. 

] 

c 

j 

j 

s_ 

2CaCO3+MgC03=CaMgCsO,+CaC03  
CaCO,  +  MitCO,-CaMK(CO»),  .. 

H 

d 

i 

d 

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H 

S 
& 

CasAl(1Si,jOa8.18H2O+2Al(OH)a+4Na.CO3=4H<NaoAl.Pi,O,.+3CaCO3+CO.-)-18H»O  
(Recrystallization)  ... 

(Reactions  variable)  .  . 

"5 

3 
f 

1 

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(MgF)2Mg3SL08+3H20=H,Mg3Si20,+Mg(OH)j+MgF., 
(Not  formulated)... 

(MgF)2Mg7Si1Oie+6H.,0=2H,Mg8Si.O!,+2Mg(OH)e+Mg:F2  .  .  .  . 
AUO,+H.O-2rAlO.(OH'jl  ... 

Alo()3+3H2O-2Al(OH)3  . 

TABLES. 


381 


la 

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Chlorite  
Epidote  
Magnetite  .  .  . 

Feldspar  
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Quartz  

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Source. 

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2Al.O34-2SiO.,4-CaCOs4-H2O-H2CaAl,Si.iOi2+CO2  

3Al.,O3+6SiO2+4CaCO34-H.O-HoCa4Al6Si6O2e4-4CO»  

3(MgFe)8iO:,4-2H.,O  =  n,(MgFe)3Si<,Oi,4-SiO2,  or] 

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382 


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TABLES. 


383 


Volume 
change. 

S 

f 

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S       S          S 
J       S         S 

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3O12.Ca3Fe2Si3O]2+5CO2+H2O=2HCa 
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ioAl2FeSi3O,3+2CaCO3+3MgCO3+3Si 
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g2FeSiA2.2[(Mg4Fe2)(Al0Fe3)Si6O36 
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.  .  Talc,  diaspore  
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4H2KMg8AlSiA2+6H,0+4C02=3H2Mg3Si4012+4Al(OH 
4H2KMgsA18isOia4-2H2O+4CO=3H2Mg3Si4O12+4AlO(OE 
2H2KMg8AlSi,0,24C024-4HaO=H2Mg3Si40124-H4Mg3Si!0 

(Not  formulated)  
HjCa^  Al2Si3Oi,  +2MgCO3  4-  H2O  =  H4Mg2Al.iSiO9+  2SiO2  +2 
4FeSj+22O4-3HsO=2Fes03.3HaO4.8SOi!  
FeS2+6O=.FeS044-SO>or  FeS2+SO+H2O=FeSO44-H2S  1 

4FeSO44-2O4-7H2O=2Fe.iO3.3H2O+4H2SO4 
3FeSo+16O-Fe3O44-6SO2  
3FeSa4-4H»O4-4O=Fe3O4+4H2S4-2SO..  

4Mg3Al2Si30,24-15H204-3C02=3H2Mg3Si4012+3MgC034-8, 
4Mg3AlsSi3O,a4-6H20=3H2Mg3Si4012+3MgAl!044-2Al(OH 
Mg3Al2Si3O124  6HaO=H4Mg3Si,,094-2Al(OH)34-Si02  
Mg3Al2Si30124-2H204-COa=H4Mg.,Al2SiO94-MgC03+2SiO, 
3Mg3Al2Si3O,j4-8H20=H16Mg9Al6Si50364-4Si02  
Mg3Al28i30,.=2MgSi034-MgAl204+SiO,  
4Fe3Al28i3O12.2Mg3Al2Si8Oi2+15HoO=3H10Fe4Mg2Al4Si4O2 
Fe3Al2Si3O12.2Mg3Al2Si3O12=3MgFeSi2O6+3MgAl2O44-3Si( 
3[2Mg3Al2Si3Oi2.Fe3Al2Si3O12.Ca3Fe2Si3Oi2]4-4CO2= 

5CaMg2FeSi4O12.2[(Mg4Fe2)(Al9Fe3)Si6036]+4CaCO3- 
Ca3Al2Si3Oi2.Mg3Al2Si3Oi2.Ca3Fe2Si3O12+H2O+5CO2=2HC 

Fft,.S,«-l-inH«S—  HFeSo4-10H0 

3FenSi2+1160=llFe3O4436SO2  or  3Fe11S12+36H2O4-8O= 
4Fe,iS,.+33HoO+162O=ll(2FenO,.3H.O)4-48SO  ... 

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)  +15H2O+3C02=3(Na2Al2Si,O122H2O)  +6Al(OH)3+4NaCl+3Na 
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TABLES.                                          ,  395 

TABLE  D. —  Classification  of  alterations,  with  volume  changes. 

INDEX  TO  CLASSIFICATION. 

Pa3.3. 

Carbonation 396 

Carbonation  and  defluoridation 396 

Carbonation  and  dehydration 396 

Carbonation,  dehydration,  and  desulphation 396 

Carbonation,  dehydration,  and  desilication 396 

Carbonation  and  deoxidation 396 

Carbonation,  deoxidation,  and  dehydration 396 

Carbonation  and  desilication 396 

Carbonation  and  hydration 397 

Carbonation,  hydration,  and  dechloridation 398 

Carbonation,  hydration,  and  desilication 398 

Carbonation,  hydration,  and  desulphation 399 

Carbonation,  hydration,  oxidation,  and  desilication 399 

Carbonation,  hydration,  and  silication 399 

Carbonation,  oxidation,  dehydration,  and  desilication 399 

Change  of  symmetry  and  molecular  change 399 

Chloridation 400 

Deboration  and  decarbonation 400 

Decarbonation 400 

Decarbonation  and  titanation 400 

Dehydration 400 

Dehydration  and  decarbonation 401 

Deoxidation 401 

Desilication 401 

Hydration 402 

Hydration  and  decarbonation 402 

H ydration  and  dechloridation 403 

Hydration,  dechloridation,  earbonation,  and  desilication 403 

Hydration,  dechloridation,  and  decarbonation 403 

Hydration  and  defluoridation 403 

Hydration  and  desilication 404 

Hydration,  desilication,  and  decarbonation 401 

Hydration  and  oxidation 404 

Hydration,  oxidation,  and  desilication 405 

Hydration  and  silication 405 

Molecular  division '. 405 

Oxidation 405 

Oxidation  and  decarbonation 405 

Oxidation,  decarbonation,  and  desulphidation 406 

Oxidation  and  desulphidation 406 

Oxidation,  hydration,  and  decarbonation 406 

Oxidation,  hydration,  and  desulphidation 406 

Oxidation  and  titanation 405 

Silication 406 

Silication  and  decarbonation ' 407 

Silication  and  dehydration 407 

Silication,  dehydration,  and  decarbonation 407 

Silication,  hydration,  and  decarbonation 408 

Silication,  oxidation,  and  decarbonation 408 

Substitution  of  bases 408 

Sulphidation 408 

Sulphidation,  deoxidation,  and  earbonation 408 


396 


A  TREATISE  ON  METAMOKPHISM. 


TABLE  D. —  Classification  of  alteration*.  >/•  it h  volume  changes — Continued. 

CARBONATION. 


Source. 


Products. 


Volume 
change. 


Brucite . 


Hydromagnesite 


CARBONATION  AND  DEFLUORIDATION. 


Per  cent. 
+  73.08 


Fluorite . 


Calcite 


CARBONATION  AND  DEHYDRATION. 


+  47.66 


Biotite i  Hypersthene,  Billimanite -  24.  68 

Laumontite \  Albite -  34.92 

| 

CARBONATION,  DEHYDRATION,  AND  DESULPHATION. 

Gypsum Calcite -  50.29 

CARBONATION,  DEHYDRATION,  AND  DESILICATION. 

Biotite Epidote,  spinel,  quartz -  14.  71 

Biotite,  hematite do -  18.15 

Serpentine Magnesite,  quartz +  18.  84 

CARBONATION  AND  DEOXIDATION. 
Magnetite j  Siderite +101.30 

CARBONATION,  DEOXIDATION,  AND  DEHYDRATION. 
Limonite Siderite +  22.  27 

CARBONATION  AND  DESILICATION. 

Almandite,  melanite,  and  pyrope Hornblende,  calcite,  quartz +  24. 55 

Grossularite Meionite,  calcite,  quartz +  54.  62 

Melanite  (see  Almandite). 
Pyrope  (see  Almandite). 

Serpentine Magnesite,  brucite,  quartz +  13.02 

Titanite Octahedrite,  oaleite,  quartz +  42.  07 

Do Rutile,  calcite,  quartz +  39.22 


TABLES. 


397 


TABLE  D. — Classification  of  alterations,  with  volume  changes — Continued. 
CARBOXATION  AND  HYDRATION. 


Source. 

Products. 

Volume 
change. 

Epistilbite,  gibbsite    .       .         ...     .  - 

Per  cent. 
+37.14 

Do                                                  - 

Heulandite,  gibbsite  

+37.  14 

Do 

Stilbite,  gibbsite 

+43.50 

Phillipsite 

+31.  98 

Phillipsite,  gibbsite 

+40.61 

Laumontite,  gibbsite                           .......... 

+33.  65 

Do                                                  . 

Scolecite,  gibbsite  

+35.23 

Biotite                                                           .     . 

+17.26 

Muscovite,  albite                   .       ..   ..  ..... 

.76 

Biotite                                                      -     ... 

Biotite-chlorite  .  .... 

+22.  92 

Do                                  

Hydrobiotite  

+  3.80 

Do 

Serpentine,  gibbsite,  kaolin                 

+14.  26 

Enstatite 

Talc  

+  9.93 

Orthoclase                                                        - 

—38.  57 

Do 

Orthoclase,  kaolin                    .   ....  

—10.58 

Do 

Orthoclase,  muscovite  .........  _.__...... 

-12.43 

Leucite,  albite,  anorthite  (see  Albite). 
Meionite                               

Kaolin,  calcite  

+35.40 

Do 

Muscovite,  calcite  ...  -  . 

+29.42 

Nephelite 

Analcite,  diaspore  

+  5.49 

Do                     

Analcite,  gibbsite  

+19.  68 

Do 

Hydronephelite.  

+23.  49 

Do 

Muscovite 

—38.  46 

Do 

Muscovite,  kaolin  .       .       

—16.  50 

Do 

Natrolite,  diaspore  

+15.00 

Do 

Natrolite,  gibbsite  

+24.  46 

Do 

Finite,  kaolin  ..  ..-. 

-13.00 

Phlogopite 

Hydrophlogopite  

+26.  89 

Do 

Talc,  diaspore  ..   

18.27 

Do 

Talc,  gibbsite  

-  7.79 

Do 

Talc  gibbsite,  serpentine  

+  5.23 

Talc,  magnesite,  gibbsite  

+75.  91 

Muscovite,  inicrocline  _. 

+31.  74 

Talc               

.83 

Do 

Talc  calcite                  .   ..........  .. 

+25.  61 

398 


A  TREATISE  ON  METAMORPHISM. 


TABLE  D.  —  Classification  of  alterations,  with  volume  changes — Continued. 
CARBONATION,  HYDRATION,  AND  DECHLORIDATION. 


Source. 

Products. 

Volume 
change. 

Sodalite 

Analcite,  diaspore  

Per  cent. 
20.77 

Do                                                  

Analcite,  gibbsite  

—10.  11 

Do                                        

Hydronephelite  

—  7.25 

Do  

Muscovite,  kaolin  

37.07 

Do  

Natrolite,  diaspora  *  

—13.  62 

Do 

Natrolite,  gibbsite 

—  6  52 

CARBONATION,  HYDRATION,  AND  DESILICATION. 


Actinolite  ...... 

Bastite 

—18  06 

Do  

Bastite,  calcite,  quartz 

+38  67 

Albite 

+  1  58 

Do 

4  89 

Albite,  anorthite 

Chabazite  gibbsite  quartz 

+46  76 

Do           ... 

Mesolite  gibbsite  quartz 

+24  96 

Do    

Mesolite,  gibbaite  quartz  calcite 

+30  19 

Anorthoclase  .  

Gibbsite 

68  02 

Do  

Gibbsite  quartz 

3  30 

Do  

Kaolin 

52  19 

Do  

Kaolin,  quartz 

9  56 

Biotite,  hematite  

Epidote,  quartz  diaspore 

18  45 

Diopside  _  

Serpentine  quartz 

-4-       44 

Do  

Serpentine,  quartz,  calcite 

+56  32 

Do  

Talc  . 

30  13 

Do  

Talc,  calcite,  quartz 

+48  74 

Epidote  

-t-fiq  ns 

Grossularite  

Zoisite,  calcite,  quartz 

+40  49 

Grossularite,  melanite  

Epidote,  calcite,  quartz 

+40  88 

Grossularite,  melanite,  pyrope  

Epidote,  calcite,  quartz,  magnesite 

+39  53 

Hornblende  

Chlorite    epidote    calcite     siderite    quart/ 

+25  39 

Melanite  (see  Grossularite). 
Orthoclase  or  microcline  

hematite. 
Gibbsite,  quartz 

6  61 

Do  

Kaolin  

54  44 

Do  

Kaolin   quartz 

1O    C7 

Do  

Muscovite,  quartz  

-15.  58 

TABLES. 


399 


TABLE  D.  —  Classification  of  alterations,  with  volume  changes — Continued. 
CAEBONATION,  HYDRATION,  AND  DESILICATION— Continued. 


Source. 


Products. 


Volume 
change. 


Pyrope 

Pyrope  (see  Grossularite). 

Sahlite 

Do 

Zoisite 


Amesite,  magnesite,  quartz 


Bastite,  quartz 

Bastite,  quartz,  calcite 

Calcite,  gibbsite,  kaolin,  quartz 


Percent. 
+62.26 

+  1.93 
+56.  41 
+66.22 


CARBONATION,  HYDRATION,  AND   DESULPHATION. 


Haiiynite Chabazite,  gibbsite -  7.  46 

Do Natrolite,  gibbsite,  calcite +  4. 99 

Do Stilbite,  gibbsite,  calcite +  .46 

Noselite Natrolite,  gibbsite —16. 44 

CARBONATION,  HYDRATION,  OXIDATION,  AND  DESILICATION. 

Actinolite Talc —36.51 

Do Talc,  calcite,  hematite,  quartz +20.33 

Augite Chlorite,  epidote,  quartz,  hematite +  8. 58 

Do Chlorite,  epidote,  quartz,  hematite,  magnesite.  +15. 43 

Olivine Serpentine,  magnetite,  magnesite,  quartz +37. 13 

Sahlite Serpentine,  magnetite,  calcite,  quartz +37. 50 

Do Talc,  magnetite,  calcite,  quartz +27.  88 

CARBONATION,  HYDRATION,  AND  SILICATION. 

Hornblende,  quartz Biotite,  calcite +41. 13 

CARBONATION,  OXIDATION,  DEHYDRATION,  AND  DESILICATION. 

Biotite Epidote,  quartz —14.  86 

CHANGE  OF  SYMMETRY  AND  MOLECULAR  CHANGE. 

Andalusite Cyanite -12.03 

Aragonite Calcite +  8.35 

Bronzite  or  hypersthene Anthophyllite • °  +  8.  70 

Marcasite..          Pyrite -2.98 


p.  gr.  hypersthene. 


400 


A  TREATISE  ON  METAMORPHISM. 


TABLE  D. — Classification  of  alterations,  with  volume  changes — Continued. 

CHLORIDATION. 


Source. 

Products. 

Volume- 
change.  . 

Albite                                                        

Marialite  

Per  cent. 
+10.  29 

Albite  halite 

do  

+  1.84 

Sodalite         

+33.  14 

Nechelite.  halite  .  . 

..do.. 

+15.64 

DEBORATION  AND  DECARBONATION. 


Biotite  

-  ti.  75 

Do 

Biotite,  gibbsite  

+  3.96 

DECARBONATION. 


Meionite 

—  3.78 

Spinel 

—29.  17 

DECARBONATION  AND  TITANATION. 


Rutile,  siderite. 


Ilmenite. 


-34.77 


DEHYDRATION. 


Albite  gibbsite                                          .   . 

Paragonite 

—18  85 

Anorthoclase,  gibbsite 

Muscovite,  paragonite 

—20  04 

Diaspore                                       -         

Corundum  

—28.  18 

Gibbsite                                

.   ...do  

—61.81 

Do                             

Diaspore  

—46.  82 

Gypsum     ....     

Anhydrite  .  . 

—37.  62 

Heulandite  

Albite  ... 

—25.  03 

Do  

Orthoclase  

—18.44 

Ijaumontite 

Analcite 

—  4  30 

Linionite                        .   . 

Hematite 

—37  78 

Opal                      

Chert,  chalcedony 

Do     

Quartz 

—22  81 

Orthoclase  or  microcline,  gibbsite  

Muscovite  (damourite) 

20  81 

Stilbite 

Albite 

31  67 

Do  

Orthoclase  

—25.66 

TABLES. 


401 


TABLE  D. — Classification  of  alterations,  with  'volume  changes — Continued. 
DEHYDRATION  AND  DECARBONATION. 


Sourci'. 

Products. 

Volume 
change. 

Analcite 

Orthoclase  prehnite 

Percent. 
14  09 

Anorthoclase,  gibbsite  

Biotite,  paragonite 

—10.  91 

Apophvllite 

Pectolite 

19  48 

Chabazite 

Natrolite 

—  4  58 

Diaspore,  magne.site....   .........   ...... 

Spinel  . 

—40.  39 

(  lililisite,  iiiagiu'siti*  

do  

—60.12 

Ijaumontite 

Ortboclase,  prehnite 

17  75 

Mesolite  

Prehnite 

—15.  05 

Natrolite  

do  

—  16.  12 

Orthoclase  or  microclint',  magnesite,  sider- 

Biotite 

—22  33 

ite,  gibbsite. 
Scolecite  

Prehnite  . 

—  16.66 

HYDRATION,  DESILICATION,  AND  DECARBONATION. 


Anorthocla^e,  calcite,  hematite 

Epidote,  quart/ 

28  30 

Biotite,  hematite  

Biotite-chlorite,  epidote,  quartz,  diaspore 

+  1  81 

Orthoclase   or    microclino,  calcite,  hema- 

Epidote, quartz  

—33.  73 

tite. 
Orthoclase  or  microcline,  magnesite,  sid- 

Biotite,  quartz  

—22.64 

erite. 

1 

JEOXIDATION. 

Hematite  

Magnetite 

2  38 

1 

)ESILICATION. 

Almandite,  pvrope  . 

Hvpersthene,  npinel  quartz 

+12  66 

Pvrope  

Knstatite,  spinel,  quartz 

+13.  51 

Titanite        

Perovskite  quartz 

+       14 

MON    XLVII — (. 


402  A  TREATISE  ON  METAMORPHISM. 

TABLE  D. — Classification  of  alterations,  with  volume  changes — Continued. 

HYDKATION. 


Source. 

Products. 

Volume 
changes. 

Kaolin  

Per  cent. 
3.  15 
+  61.87 
+_  60.30 
+  52.76 
+  34.65 
7.77 

Do 

Kaolin,  gibbsite                

Gypsum 

Gismondite 

Do                                           

Thomson!  te  

Do 

Zoisite,  kaolin 

Anorthite  hematite                                .... 

Epidote,  kaolin,  gibbsite  

+     3.60 
+     8.  64 
+  39.25 
+161.  83 
+  10.  11 
+  84.01' 
+  60.72 
.86 

Cancrinite                                                   -   -  - 

Natrolite,  gibbsite,  calcite  

Diaspore 

Do 

Gibbsite 

Kaolin 

Do                         ..... 

Kaolin,  gibbsite  

Hematite                                  .               ... 

Limonite  

lolite  (  cordierite)                                 -   - 

Chlorophyllite  

Leucite                            '        ...       .  .. 

Analcite  

+  10.  74 
1.62 

Meionite,  hematite                   .          ...... 

Epidote,  gibbsite  

Nephelite          .       .                . 

Thomsonite  

+  24.60 

+  36.84 

Pvrone 

Talc,  spinel,  gibbsite  

Serpentine.     ..   .  .  .   . 

Webskyite 

Sillimanite 

Kaolin                                                                                1  47 

Do  

Kaolin,  gibbsite        .   . 

+  64.67 

+  24.05 

Zircon  

Malacon  (hydrous  zircon) 

HYDRATION  AND  DECARBONATION. 

Andalusite 

Talc  (steatite) 

32  37 

Do  

Talc,  gibbsite 

+  97.67 
9  55 

Do  

Muscovite  (damourite) 

Do  

Muscovite,  gibbsite 

+  55.47 
+  22.92 
23  12 

Biotite  

Chlorite 

Cyanite  

Talc  (steatite) 

Do  

Talc,  gibbsite 

+124.  71 
+     2.83 
+  76.74 
+  16.  50 
+  88.44 
-  25.23 
+  46.69 

Do.. 

Muscovite  (H^mniirite) 

Do  *  

Muscovite,  gihhsite 

Muscovite  

Serpentine 

Do  

Do  

Talc 

Do.. 

Talc,  gibbsite  .  . 

TABLES. 


403 


TABLE  D.  —  Classification  nf  alterations,  with  volume  change — Continued. 
HYDRATION  AND  DECARBONATION— Continued. 


Source. 


Products. 


Volume 
change. 


Per  cent. 

Phlogopite Chlorite 4  41. 02 

Sillimanite Muscovite  (damourite)  7.  98 

Do Muscovite,  gibbsite +  58. 16 

Do Talc  (steatite) -  31. 20 

Do Talc,  gibbsite +101. 09 

Staurolite..                   Chlorite  (amesite),  gibbsite +103.58 


HYDRATION  AND  DECHLORIDATION. 


Sodalite..  ..:...? Thomsonite 


-  v  6. 41 


HYDRATION,  DECHLORIDATION,  CARBONATION,    AND  DESILICATION. 


Marialite Muscovite,  quartz 


HYDRATION,  DECHLORIDATION,  AND  DECARBONATION. 


-  16.74 


Marialite  . .  Kaolin,  talc 


7.69 


HYDRATION  AND  DEFLUORIDATION. 


Chondrodite Serpentine,  brucite. 

Clinohumite do 

Humite  . .  do 


+  30.15 
+  38.39 
-(-  35.53 


404 


A  TREATISE  ON  METAMORPHISM. 


TABLE  D.  —  Classification  of  alterations,  with  volume  changes — Continued. 
HYDRATION  AND  DESILICATION. 


Source. 

Products. 

Volume 
change. 

Albite                                                    

Analcite,  quartz  

Per  cent. 
+20.  82 

Do                                                    

Natrolite,  quartz  

+19.  95 

Albite  anorthite  orthoclase 

Albite,  zoisite,  muscovite,  quartz  

Aphrosiderite,  quartz  

+50.98 

Zoisite,  gibbsite,  quartz  

—  4.58 

Do                                                

Prehnite,  gibbsite,  quartz  

+14.  85 

Anorthite  hematite                         -  .  .  . 

Epidote,  gibbsite,  quartz  .         

+  6.57 

Anthophvllite                                         ...   . 

Bastite  

+12.09 

Do                                       ... 

Bastite,  quartz  

+34  09 

Bronzite  or  hypersthene                     .... 

Bastite  ,» 

°+22.  77 

Do 

Bastite,  quartz 

&+15.  65 
o  _(-46  87 

Cummingtonite 

Bastite 

+f4  20 

Do 

Bastite,  quartz 

+36  76 

Enstatite                                  .         ... 

Serpentine 

+  14  25 

Do 

Serpentine,  quartz 

+38  36 

Orthoclase  (we  Albite). 
Prehnite 

Chlorite,  quartz 

+  3  27 

Pyrope 

do 

+56  02 

Do 

Serpentine,  gibbsite  quartz 

+81  61 

Pyrope  (see  Almandite). 
Serpentine                          ..       ... 

Brucite,  quart/ 

+  9  82 

HYDRATION  AND  OXIDATION. 


Anthophyllite  

Talc,  hematite  

+11.41 

Do  

Talc,  limonite 

Bronzite  or  hvpersthene  

Talc,  hematite  

Do  

Talc,  limonite 

Do  

Talc,  magnetite 

-»-|-14  68 

* 
Magnetite  

Limonite  . 

"  +21.  73 
«+18.  20 
+64  63 

Olivine  

Serpentine,  magnetite 

+29  96 

a  Sp.  gr.  hypersthene. 


6Sp.  gr.  bronzite. 


c  Average  sp.  gr.  bronzite  and  hypersthene. 


TABLES. 


405 


TABLE  D. — Classification  of  alterations,  with  volume  changes — Continued. 
HYDRATION,  OXIDATION,  AND  DESILICATION. 


Source. 


Products. 


Bronzite  or  hypersthene Serpentine,  hematite 

Do Serpentine,  hematite,  quartz. . 

Hypersthene Talc,  magnetite,  quartz 

Do j  Serpentine,  magnetite,  quartz . 

Olivine do 

Do do 

HYDRATION  AND  SILICATION. 

Hornblende,  quartz Biotite,  epidote 

MOLECULAR  DIVISION. 

Spodumene  (beta-spodumene) Eucryptite,  albite 

OXIDATION. 

Ilinenite Octahedrite,  hematite 

Do Octahedrite,  magnetite 

Do Rutile,  hematite 

Do Rutile,  magnetite 

Magnetite Hematite 

OXIDATION  AND  TITANATION. 

Rutile,  magnetite Ilmenite,  hematite 

OXIDATION  AND  DECARBONATION. 

Ankerite Hematite 

Do Magnetite 

Parankerite Hematite 

Do Magnetite 

Siderite Hematite 

Do Magnetite 


«  Sp.  gr.  hyp^rsthene. 


406 


A  TREATISE  ON  METAMORPHISM. 


TABLE  D. — Classijfioation  of  alterations,  with  volume  changes — Continued. 
OXIDATION,  DECARBONATION,  AND  DESULPHIDATION. 


Source. 


Products. 


Volume 
change. 


Per  cent. 

Siderite,  raarcasite Magnetite —47. 14 

Siderite,  pyrite do —46. 67 

| 

OXIDATION,  HYDRATION,  AND  DECARBONATION. 

Ankerite Limonite 

Meionite Epidote,  gibbsite +  7. 55 

Parankerite Limonite 

Siderite do -18. 22 

Staurolite Muscovite  (damourite) —24. 96 

Do Muscovite,  magnetite,  gibbsite +68. 08 

Do v Talc 44.02 

Do Talc,  gibbsite +90.  96 

OXIDATION,  HYDRATION,  AND  DESULPHIDATION. 

Marcasite Limonite -  0. 14 

Pyrite do +  2.93 

Pyrrhotite.v do +24.68 

j . 

OXIDATION  AND  DESULPHIDATION. 

Marcasite Magnetite —39.34 

Pyrite do —37. 48 

Pyrrhotite do —24. 27 

SILICATION. 

Corundum,  quartz Cyanite -  6. 59 

Do Sillimanite ; +  4.38 

Gehlenite Grossularite -  4. 42 

Gehlenite,  quartz do —18.56 

Olivine,  quartz Anthophyllite -  1. 48 

Nephelite,  quartz Albite •     .41 


TABLES. 


407 


TABLE  D. — Classification  of  alterations,  with  volume  changes — Continued. 
SILICATION  AND  DECARBONATION. 


Source. 


Products. 


Volume 
change. 


Per  cent. 

Ankerite  or  parankerite,  quartz Actinolite —32. 72 

Do Actinolite,  calcite —22. 62 

Do Sahlite —37.27 

f  o_io.77 

Bronzite  or  h ypersthene,  calcite,  quartz . .    Actinolite •!  ft 

Calcite !  Wollastonite +10.81 

Calcite,  quartz I do —31.48 

Dolomite Diopside +  2.03 

Dolomite,  quartz do —40.11 

Dolomite Tremolite,  calcite -• +  9. 89 

Dolomite,  quartz do —25. 20 

Dolomite Tremolite,  wollastonite +14. 00 

Dolomite,  quartz I do —33.09 

Forsterite,  calcite,  quartz Tremolite —12. 29 

Olivine,  calcite,  quartz Actinolite —13. 34 

Rutile,  calcite,  quartz Titanite —28.17 

Siderite,  quartz Grunerite  .' —32. 53 

SILICATION  AND  DEHYDRATION. 

Analcite,  quartz Albite —17. 25 

Diaspora,  quartz '  Cyanite —22. 61 

Do :  Sillimanitev -13.52 

Gibbsite,  quartz Cyanite —49. 61 

Do Sillimanite , -43.68 

SILICATION,  DEHYDRATION,  AND  DECARBONATION. 

Diaspore,  quartz,  calcite Margarite —14.08 

Do Zoiaite —29. 44 

Diaspore,  quartz,  K..CO, Muscovite —54. 21 

Gibbsite,  quartz,  calcite Margarite —38.92 

Do Zoisite -43.06 

Gibbsite,  quartz,  K2CO3 Muscovite -64.99 


<"  Sp.  gr.  bronzite. 


l>Sp.  gr.  hyperathene. 


408 


A  TREATISE  ON  METAMOKPHISM. 


TABLE  D. — Classification  of  alterations,  with  volume  changes — Continued. 
SILICATION,  HYDRATION,  AND  DECARBONATION. 


Source. 


Products. 


Volume 
change. 


Per  cent. 

Corundum Margarite 

Corundum,  quartz,  calcite do 1.22 

Corundum Zoisite •. +261. 34 

Corundum,  quartz,  calcite do -  23.58 

Corundum ,  Muscovite  (damourite) +264. 25 

Corundum,  quartz,  K2COS ^ do +     1.62 

SILICATION,  OXIDATION,  AND  DECARBONATION. 

Ilmenite Titanite +  76. 35 

Ilmenite,  calcite,  quartz Titanite,  magnetite —  22.  35 

SUBSTITUTION  OF  BASES. 

Augite Hornblende +    4. 30 

Augite,  siderite,  magnesite '  Hornblende,  calcite +    6. 14 

Calcite Dolomite —  12. 30 

Diopside Tremolite +    5.68 

Diopside,  magnesite Tremolite,  calcite +  10. 55 

Hornblende Augite —    4.13 

Leucite Orthoclase,  nephelite 7. 59 

Muscovite • Paragonite —     2.67 

Olivine,  anorthite Actinolite,  spinel —     7.18 

Sahlite Actinolite +     7.28 

Sahlite,  siderite,  magnesite Actinolite,  calcite +  10. 81 

Spodumene Beta-spodumene +  24.  72 

SULPHIDATION. 

Pyrrhotite Pyrite 4.  21.13 

. • : .                              j  « 

SULPHIDATION,  DEOXIDATION,  AND  CARBONATION. 

Hematite Marcasite,  siderite +  78.  73 

Do Pyrite,  siderite _|_  76. 12 


Rec'a  UCB 

JUL181986 


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